Rethink validation error representation

Currently this is a pretty slim wrapper for Everit's `ValidationException`, and there are probably some ways it could be improved. For a document with two schema violations, for example, there are three error values, since the top-level Everit exception ("2 schema violations found") is included.

The issue names no files or tests; start by tracing the wrapper around Everit’s ValidationException and how its errors are exposed. Define the expected representation for a document with two schema violations, then update the relevant implementation and tests once that behavior is agreed.

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