Better error reporting on unsupported keywords

[fredbi]

Unsupported json keywords in swagger are a common failure for my specs which are largely inherited from previously designed json schemas.

Currently, error reporting is terse on this kind of problem.

At best, I have a "forbidden property xxx" message.
At worse, I have a "xxx does not validate oneOf ..." message.

The switch between these 2 types of message is dificult to control (related to MatchCount, which makes a best effort to provide correct reporting on anyOf|oneOf failures).

I think an additional scan with a list of json keywords explicitly not supported by swagger (e.g: $schema, additionalItems, ...) would provide the user a clearer view on what's wrong with the spec.