Refine error handling text (#816)

* Refine error handling text

* Apply suggestions from code review

Co-authored-by: Addison Phillips <[email protected]>

* Update fallback text

* Turn bullet point list into paragraphs

* Be more mighty

Co-authored-by: Addison Phillips <[email protected]>

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Co-authored-by: Addison Phillips <[email protected]>

Author: eemeli

spec/errors.md

@@ -24,9 +24,22 @@ or _Message Function Errors_ in _expressions_ that are not otherwise used by the
 such as _placeholders_ in unselected _patterns_
 or _declarations_ that are never referenced during _formatting_.
 
-In all cases, when encountering a runtime error,
-a message formatter MUST provide some representation of the message.
-An informative error or errors MUST also be separately provided.
+When formatting a _message_ with one or more errors,
+an implementation MUST provide a mechanism to discover and identify
+at least one of the errors. 
+The exact form of error signaling is implementation defined.
+Some examples include throwing an exception,
+returning an error code, 
+or providing a function or method for enumerating any errors.
+
+For all _messages_ without _Syntax Errors_ or _Data Model Errors_,
+an implementation MUST enable a user to get a formatted result.
+The formatted result might include _fallback values_ 
+such as when a _placeholder_'s _expression_ produced an error
+during formatting.
+
+The two above requirements MAY be fulfilled by a single formatting method,
+or separately by more than one such method.
 
 When a message contains more than one error,
 or contains some error which leads to further errors,

spec/formatting.md

@@ -741,9 +741,8 @@ each _text_ and _placeholder_ part of the selected _pattern_ is resolved and for
 
 Resolved values cannot always be formatted by a given implementation.
 When such an error occurs during _formatting_,
-an implementation SHOULD emit an appropriate _Message Function Error_ and produce a
-_fallback value_ for the _placeholder_ that produced the error.
-A formatting function MAY substitute a value to use instead of a _fallback value_.
+an implementation MUST emit an appropriate _Message Function Error_ and use a
+_fallback value_ for the _placeholder_ with the error.
 
 Implementations MAY represent the result of _formatting_ using the most
 appropriate data type or structure. Some examples of these include: