Modify TagHelperDescriptorResolver and dependencies to not throw.

- Changed the TagHelperDescriptorResolver, TagHelperTypeResolver and AddOrRemoveTagHelperSpanVisitor to not throw when they're unable to understand the users directive lookup text.
- This involved utilizing the new ParserErrorSink to capture errors found during TagHelperDescriptor resolution.

#210
Add and modify tests to validate TagHelperDescriptorResolver doesn't throw.

- Modified existing tests that expected the resolvers to throw to no longer throw.
- Added new test to validate that unexpected errors that are thrown are also handled.

#210
Changed TagHelper attributes to be SpanKind.Code if not string typed.

- Also added tests to validate that non string TagHelper attributes inherit the SpanKind.COde behavior.
- Removed a block wrapping around single markup spans.