Convert database constraint errors to validation errors

This issue is a (choose one):

• Problem/bug report.
• Feature request.
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

Currently, when an error comes from the database, the only way to handle it is to use on_server_error :first_callback. This is not easy to use when we are only talking about invalid attributes.

Features:

Sometimes, db constraints are more efficient that validation. Then why not transform database constraint error into validation error.

Start by tracing the existing on_server_error :first_callback path and how database errors are currently surfaced by the Rails library. Determine which database constraint errors should become validation errors and what the resulting JSON:API error behavior should be; done means the behavior is defined and covered by the relevant tests.