Feature request - replace `errors` with problems; reuse/extend problem schema more

[DavidBiesack]

This is a suggestion for a feature request.
The current schema defines an errors array using a secondary error-item.yaml schema.
I suggest two changes:

1. rename errors to problems, since all errors are problems, but not all problems are errors. That is, this is application/problem+json not application/error+json
2. instead of introducing another data models for the array items, just reuse (or extend) the existing schemas/problem-details.yaml schema.
   • replace code with type as in problem-details.yaml, for the same reason that type uses a URI and not just a string/code.
   • replace pointer and header and parameter with the instance property to be the uri-reference of the specific API request that has a problem. This can be a URN (a URN is one form of a URI); the prefix on the URN indicates what type of value the problem relates to, analogous to(but broader than) the in values for a parameter in OpenAPI. Values of in tell the recipient how to interpret or navigate the instance: URI. Values are body:, header:, path: and the remainder of the URN can be (in URN format) the values indicated for the existing pointer and header and parameter values in error-item.yaml.

If you like, I can draft a PR which more completely demonstrates this, with examples.