Fix jsonschema errors

[glasserc]

#1245 changed the format of the jsonschema error messages. It isn't clear exactly why; the commit message just says "Simplify and fix coverage". https://github.com/Kinto/kinto-attachment has a test that started failing because it relies on the format of the error message. Restore it.

• (n/a) Add documentation.
• Add tests.
• Add a changelog entry.
• (n/a) Add your name in the contributors file.
• (n/a) If you changed the HTTP API, update the API_VERSION constant and add an API changelog entry in the docs

r? @leplatrem

[leplatrem]

It isn't clear exactly why; the commit message just says "Simplify and fix coverage"

There was a piece of code that was never executed. Since no test was failing without it, I just removed it :)

[leplatrem]

Since #1245 was released, we'll need a changelog entry