Improve validation errors in mixin

[sjamaan]

This has several improvements to how validation errors are raised by the mixin:

• Use the Django built-in validation error generation to produce better errors:
  • The name of the field that triggered the validation error is used as the error dictionary key if it's a single field. This helps to highlight the field in a frontend.
  • For JSON-based APIs the code field is very useful to determine what is wrong with that particular field, and we rely on this in our project at work
• When there are failing "standard" uniqueness checks, do not bail out early but merge partial unique validation errors into the standard errors.
• When there are multiple partially unique indexes that trigger a uniqueness error, all errors are returned instead of just the first.

This builds on #15 so please take a look at that one first.

[sjamaan]

Why no feedback on this PR?

I'm relying on this code in production, and it's essential for us, in case you're doubting the use of this.