[Issue 645] Restructure error response format

[chouinar]

Summary

Fixes #645

Time to review: 20 mins

Changes proposed

Validation errors from Marshmallow are now formatted into the errors list on the response object.

Context for reviewers

I know this is a lot, it is a bit difficult to split up, and I'm happy to walk you through it.

There are a few high-level fundamental changes:

1. Each of the Marshmallow schema + fields + validators needed to be restructured a little to allow for validation error type + a message.
2. An error processor was attached to the application to restructure the Marshmallow errors
3. All error scenarios should go through the error processor and be re-formatted if necessary (mainly just Marshmallow at the moment).

Additional information

A lot of tests focused on testing these changes, including a few test-only endpoints that validate various error

The endpoints still work properly locally, and more-or-less are unmodified outside of a few small edge cases that the new field classes now account for (nullability).