Throw exception on invalid resource annotation
Throw exception on invalid resource annotation

If a resource_reference annotation is specified for a field of any
type other than string, we should throw an exception rather than
generating invalid code. This should allow presubmits to fail,
avoiding broken protos to be checked in.

Fixes #333.

Tested by:

- Demonstrating that the current Dialogflow protos fail generation
- Generating all APIs after fixing Dialogflow: no changes to current results

googleapis/gapic-generator-csharp,googleapis/gapic-generator-csharp