Before version 4.8, jsonschema did some wild guessing when producing
error messages for schemas with several equivalent subschemas. In
version 4.8 it is no longer done, causing error messages that are more
correct but also more generic.
This change restores guessing the potential root cause without claiming
that it's the only possible root cause. Also the traits schema is
simplified to make it less ambiguous.
See https://github.com/python-jsonschema/jsonschema/issues/991 for details.
Change-Id: Ia75cecd2bfbc602b8b2b85bdda20fdc04c5eadf4
62f9c61ae6