APIView doesn't show API's error model

An API's error model - i.e. how errors are communicates, what exceptions are thrown, etc - are part of the public-facing API.

It's common for a feature crew to forget to think about this until the rest of the API is designed, often resulting in a bit of churn after a first preview.

It would be ideal to represent this somehow in APIView. It would have the following advantages:
- Help feature crews think of the error design as first class, same as client and method design
- Promote conversation around whether the error model is the right one for the API, idiomatic, etc.
- Help vet that errors/exceptions are useful and helpful to customers
- Be able to see whether exception model changes between APIView diffs to help prevent API breaking changes

Start by locating APIView and understanding how it currently represents public API elements and APIView diffs. Define how an API's error model should be represented and compared, then confirm that error-model changes are visible in reviews without breaking existing API presentation.

Written by the indexing model from the issue text.