going to refactor:
- generate one `nkError` node
- contains a set of issue markers
- can generate more than one diagnostic message

Still not happy with the API and a few things are vague, to think about:
- `semIdentVis` muddles AST sem, and evaling a symbol constructor
- symbol construction wants to succeed regardless (for nimsuggest/check)
- AST needs to capture the errors (for `sempass2` output)
- how to keep sem idempotent with pragma/export AST sticking around?
  - maybe `semAfterMacroCall` can wrap output in `nkError`?
- the getSym/Recover proc feel off

Teasing this apart feels unnecessarily difficult, but perhaps it'll have
large knockon effects to various parts of sem. Especially when it comes
to clarifying semanticizing AST, symbolic, and type parts of sem.

…identvis

will clean-up later, but one takeaway is symbol producing `newSymG` that
places errors in the `.ast` field. That seems potentially cleaner.