Interpret JSON schema validator errors

[beckjake]

As part of #820, I implemented an attempt to clean up the schema validation error output. It was pretty messy and involved a bunch of regex matches so we removed it.

Currently missing attributes each are a separate message, and extra attributes get combined into one. There is an actual object involved that lets you identify if it's an error about attributes being extra vs missing, but what attribute is extra or missing is only available as part of the error text itself. It would be nice to have it be consistent, one way or the other.

[drewbanin]

we don't use json schemas directly anymore and we do a pretty good job of rendering these error messages these days :)