Improve `ASTGenError` messages

[ahoppen]

We should make a couple of improvements to the error messages of the ASTGenError types.

• The error messages should be single-line
• The error messages should tell the user to file a bug report since this is an internal compiler inconsistency
• The error messages should be somewhat actionable, e.g. UnexpectedTokenKindError should tell the developer which token kinds are acceptable.

[mininny]

I'd like to work on this if it's okay. I think the first two tasks are pretty straightforward.
For the third task, at ASTGenvisitor.visit(node: ImportDeclSyntax) it looks like the diagnostic fires when the tokenkind is not a type of BridgedImportKind. In this case, should I add a diagnostic explanation that the token should be a type of importKind, for example?

[ahoppen]

I think it would be fine to just tackle the first two points in a PR. I think the third point might be easier to do after we have implemented #68356 and thus have a list of tokens that we’re expecting from the TokenSpecSet.