Error fixes for bindings (payjoin#610)

This implements/derives common traits for errors that are useful for
language bindings.

Author: spacebear21

payjoin/src/receive/error.rs

@@ -72,6 +72,7 @@ pub enum ReplyableError {
 ///     "message": "Human readable error message"
 /// }
 /// ```
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub struct JsonReply {
     /// The error code
     error_code: ErrorCode,

payjoin/src/send/error.rs

@@ -305,7 +305,17 @@ impl ResponseError {
     }
 }
 
-impl std::error::Error for ResponseError {}
+impl std::error::Error for ResponseError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use ResponseError::*;
+
+        match self {
+            WellKnown(error) => Some(error),
+            Validation(error) => Some(error),
+            Unrecognized { .. } => None,
+        }
+    }
+}
 
 impl From<WellKnownError> for ResponseError {
     fn from(value: WellKnownError) -> Self { Self::WellKnown(value) }
@@ -376,6 +386,8 @@ impl WellKnownError {
     }
 }
 
+impl std::error::Error for WellKnownError {}
+
 impl core::fmt::Display for WellKnownError {
     fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
         match self.code {