Clarify error handling and fix normative requirements

[Addison Phillips]

In this comment I noted that the current spec contains untestable normative requirements about certain types of error (such as emitting syntax/data model errors "as early as possible"). My proposal includes changing the text:

- During selection and formatting,
- _expression_ handlers MUST only emit _Message Function Errors_.
+ _Syntax Errors_ and _Data Model Errors_ can always be detected during message parsing or processing.
+ (Other categories of error can sometimes be detected using validation tools.)
+
+ _Resolution Errors_ can be detected during _variable_, _function_, or _markup_ resolution.
+
+ _Message Function Errors_ apply to _selection_ or _formatting_
+ and include errors detected during the evaluation of an
+ _expression_ or during the execution of a _function_.