Make IntegrityError available from django.db

[enlight]

This is related to ticket #3083, however the issue I'd like to address is just a subset of that ticket, so I figured it's best to create a separate ticket. The attached patch will allow developers to import IntegrityError from django.db thus avoiding a dependency on any specific db module. I mentioned the rationale for this patch in ​this thread on django-users.

I have tested the changes on the postgresql and postgresql_psycopg2 backends, and I've looked at api/source for the oracle, mysql, ado_mssql and sqlite3 db modules recommended for use with the other backends. From what I could gather they all provide an IntegrityError as specified by the ​Python DB-API 2.0... still would be best if someone actually tested all those backends :)

[Simon G. <dev@…>]

I'm +1 - it does make sense to make this database independant.

[Michael Radziej <mir@…>]

Simon, what's missing for "Ready for Checkin"? Do you think there should be docs?

[Simon G. <dev@…>]

Hi Michael, I think I was being overly cautious :) I like the idea, but thought that one of the core's might have comments to make. I'll shift it to RFC.

[Malcolm Tredinnick]

Ticket #1515 contains a potentially better way to do this, although ​my comment there explains why I'm not committing that yet.

[Malcolm Tredinnick]

(In [5076]) Fixed #3450 -- Exposed IntegrityError in a backend-neutral fashion. This is a
useful error to be able to catch explicitly. Thanks, enlight.