Add a distinctive "code" to all ValidationErrors

[brunobord]

As the Django documentation suggests, Django ValidationErrors exceptions should

Provide a descriptive error code to the constructor

This was a "django-only" feature, and was not compatible with DRF errors embedded in the version <=3.3.

When the #239 will be closed, we'll have to make a pass on every ValidationError raised in our code, and provide the appropriate code.