use type name in Display impl for errors with no other message set

Otherwise, with nested errors, you can end up with unhelpful errors like
this (example printed by `anyhow`):

```
Error: failed to get metadata for "/temp/spoopadoop"

Caused by:
    0: Dropbox API endpoint returned an error: There is nothing at the given path.
    1: There is nothing at the given path.
    2: There is nothing at the given path.
```

instead, with this change, you get:
```
Error: failed to get metadata for "/temp/spoopadoop"

Caused by:
    0: Dropbox API endpoint returned an error: GetMetadataError: There is nothing at the given path.
    1: GetMetadataError: There is nothing at the given path.
    2: There is nothing at the given path.
```
which is less repetitive and gives a better indication of what actually went wrong.

Author: wfraser

generator/rust.stoneg.py

@@ -1144,6 +1144,9 @@ def _impl_display(self, typ: ir.DataType) -> None:
                             msg = msg[:-1]
                         if msg:
                             msg += ': '
+                        else:
+                            # if there's no message for the variant, use the type name
+                            msg = f"{type_name}: "
                         msg += inner_fmt
                         args = 'inner'
 

src/generated/types/file_properties.rs

@@ -301,7 +301,7 @@ impl ::std::fmt::Display for AddPropertiesError {
         match self {
             AddPropertiesError::TemplateNotFound(inner) => write!(f, "Template does not exist for the given identifier: {:?}", inner),
             AddPropertiesError::RestrictedContent => f.write_str("You do not have permission to modify this template."),
-            AddPropertiesError::Path(inner) => write!(f, "{}", inner),
+            AddPropertiesError::Path(inner) => write!(f, "AddPropertiesError: {}", inner),
             AddPropertiesError::UnsupportedFolder => f.write_str("This folder cannot be tagged. Tagging folders is not supported for team-owned templates."),
             AddPropertiesError::PropertyFieldTooLarge => f.write_str("One or more of the supplied property field values is too large."),
             AddPropertiesError::DoesNotFitTemplate => f.write_str("One or more of the supplied property fields does not conform to the template specifications."),
@@ -911,7 +911,7 @@ impl ::std::fmt::Display for InvalidPropertyGroupError {
         match self {
             InvalidPropertyGroupError::TemplateNotFound(inner) => write!(f, "Template does not exist for the given identifier: {:?}", inner),
             InvalidPropertyGroupError::RestrictedContent => f.write_str("You do not have permission to modify this template."),
-            InvalidPropertyGroupError::Path(inner) => write!(f, "{}", inner),
+            InvalidPropertyGroupError::Path(inner) => write!(f, "InvalidPropertyGroupError: {}", inner),
             InvalidPropertyGroupError::UnsupportedFolder => f.write_str("This folder cannot be tagged. Tagging folders is not supported for team-owned templates."),
             InvalidPropertyGroupError::PropertyFieldTooLarge => f.write_str("One or more of the supplied property field values is too large."),
             InvalidPropertyGroupError::DoesNotFitTemplate => f.write_str("One or more of the supplied property fields does not conform to the template specifications."),
@@ -1624,7 +1624,7 @@ impl ::std::fmt::Display for PropertiesError {
         match self {
             PropertiesError::TemplateNotFound(inner) => write!(f, "Template does not exist for the given identifier: {:?}", inner),
             PropertiesError::RestrictedContent => f.write_str("You do not have permission to modify this template."),
-            PropertiesError::Path(inner) => write!(f, "{}", inner),
+            PropertiesError::Path(inner) => write!(f, "PropertiesError: {}", inner),
             PropertiesError::UnsupportedFolder => f.write_str("This folder cannot be tagged. Tagging folders is not supported for team-owned templates."),
             _ => write!(f, "{:?}", *self),
         }
@@ -1983,7 +1983,7 @@ impl ::std::error::Error for PropertiesSearchError {
 impl ::std::fmt::Display for PropertiesSearchError {
     fn fmt(&self, f: &mut ::std::fmt::Formatter<'_>) -> ::std::fmt::Result {
         match self {
-            PropertiesSearchError::PropertyGroupLookup(inner) => write!(f, "{}", inner),
+            PropertiesSearchError::PropertyGroupLookup(inner) => write!(f, "PropertiesSearchError: {}", inner),
             _ => write!(f, "{:?}", *self),
         }
     }
@@ -3300,9 +3300,9 @@ impl ::std::fmt::Display for RemovePropertiesError {
         match self {
             RemovePropertiesError::TemplateNotFound(inner) => write!(f, "Template does not exist for the given identifier: {:?}", inner),
             RemovePropertiesError::RestrictedContent => f.write_str("You do not have permission to modify this template."),
-            RemovePropertiesError::Path(inner) => write!(f, "{}", inner),
+            RemovePropertiesError::Path(inner) => write!(f, "RemovePropertiesError: {}", inner),
             RemovePropertiesError::UnsupportedFolder => f.write_str("This folder cannot be tagged. Tagging folders is not supported for team-owned templates."),
-            RemovePropertiesError::PropertyGroupLookup(inner) => write!(f, "{}", inner),
+            RemovePropertiesError::PropertyGroupLookup(inner) => write!(f, "RemovePropertiesError: {}", inner),
             _ => write!(f, "{:?}", *self),
         }
     }
@@ -3971,12 +3971,12 @@ impl ::std::fmt::Display for UpdatePropertiesError {
         match self {
             UpdatePropertiesError::TemplateNotFound(inner) => write!(f, "Template does not exist for the given identifier: {:?}", inner),
             UpdatePropertiesError::RestrictedContent => f.write_str("You do not have permission to modify this template."),
-            UpdatePropertiesError::Path(inner) => write!(f, "{}", inner),
+            UpdatePropertiesError::Path(inner) => write!(f, "UpdatePropertiesError: {}", inner),
             UpdatePropertiesError::UnsupportedFolder => f.write_str("This folder cannot be tagged. Tagging folders is not supported for team-owned templates."),
             UpdatePropertiesError::PropertyFieldTooLarge => f.write_str("One or more of the supplied property field values is too large."),
             UpdatePropertiesError::DoesNotFitTemplate => f.write_str("One or more of the supplied property fields does not conform to the template specifications."),
             UpdatePropertiesError::DuplicatePropertyGroups => f.write_str("There are 2 or more property groups referring to the same templates in the input."),
-            UpdatePropertiesError::PropertyGroupLookup(inner) => write!(f, "{}", inner),
+            UpdatePropertiesError::PropertyGroupLookup(inner) => write!(f, "UpdatePropertiesError: {}", inner),
             _ => write!(f, "{:?}", *self),
         }
     }