A better exception for 'throwOnUnresolved'

[matanlurey]

Instead of a StateError, something that can be caught:

import 'package:analyzer/dart/element/element.dart';
import 'package:source_span/source_span.dart';

abstract class UnresolvableAnnotationException implements Exception {
  /// Element that was annotated with something we could not resolve.
  Element get annotatedElement;

  /// Source span of the annotation that was not resolved.
  SourceSpan get annotationSource;
}

So instead of a user seeing:

Bad state: Could not resolve @null. 
An import or dependency may be missing or invalid.

... we could present, for example, as a warning in AngularDart:

[WARNING] Unresolvable annotation in /some/dir/your_file.dart.

Could not resolve the "@Mispelled()" annotation on the following element:

class Foo {}
      ^^^

You may have an invalid or misspelled import or dependency.

/cc @natebosch @jakemac53 for sanity check. This is technically a breaking change (we would no longer throw StateError). I could have this implement/extend StateError, though I guess in practice we tell users not to catch errors?

[natebosch]

SGTM. I don't think code is likely relying on the specific type now.