Backport of 680a0f08 from master.

Overriding the error messages now works for both unique fields, unique_together
and unique_for_date.

This patch changed the overriding logic to allow customizing NON_FIELD_ERRORS
since previously only fields' errors were customizable.

Refs #20199.

Thanks leahculver for the suggestion.

Thanks Keryn Knight for the suggestion.

Thanks Daniele Procida for the suggestion and edits.

Use __cause__ to expose the underlying database exceptions even
under python 2.

SuspiciousOperations have been differentiated into subclasses, and
are now logged to a 'django.security.*' logger. SuspiciousOperations
that reach django.core.handlers.base.BaseHandler will now return a 400
instead of a 500.

Thanks to tiwoc for the report, and Carl Meyer and Donald Stufft
for review.

Refs #20104.

Squashed commit of the following:

commit 2181d833ed1a2e422494738dcef311164c4e097e
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Wed Feb 27 14:28:39 2013 +0100

    Fixed #15901 -- Wrapped all PEP-249 exceptions.

commit 5476a5d93c19aa2f928c497d39ce6e33f52694e2
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 17:26:52 2013 +0100

    Added PEP 3134 exception chaining.

    Thanks Jacob Kaplan-Moss for the suggestion.

commit 9365fad0a650328002fb424457d675a273c95802
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 17:13:49 2013 +0100

    Improved API for wrapping database errors.

    Thanks Alex Gaynor for the proposal.

commit 1b463b765f2826f73a8d9266795cd5da4f8d5e9e
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 15:00:39 2013 +0100

    Removed redundant exception wrapping.

    This is now taken care of by the cursor wrapper.

commit 524bc7345a724bf526bdd2dd1bcf5ede67d6bb5c
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 14:55:10 2013 +0100

    Wrapped database exceptions in the base backend.

    This covers the most common PEP-249 APIs:
    - Connection APIs: close(), commit(), rollback(), cursor()
    - Cursor APIs: callproc(), close(), execute(), executemany(),
      fetchone(), fetchmany(), fetchall(), nextset().

    Fixed #19920.

commit a66746bb5f0839f35543222787fce3b6a0d0a3ea
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Feb 26 14:53:34 2013 +0100

    Added a wrap_database_exception context manager and decorator.

    It re-throws backend-specific exceptions using Django's common wrappers.

Added -n to sphinx builds to catch issues going forward.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16983 bcc190cf-cafb-0310-a4f2-bffc1f526a37

This allows to make these links more resilent to changes in the target URLs.
Thanks Jannis for the report and Aymeric Augustin for the patch.

Fixes #16586.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16720 bcc190cf-cafb-0310-a4f2-bffc1f526a37

Corrected a number of mislinked crossrefs and added a few new crossref targets found in/via the 1.3 release.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15894 bcc190cf-cafb-0310-a4f2-bffc1f526a37

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15733 bcc190cf-cafb-0310-a4f2-bffc1f526a37

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14329 bcc190cf-cafb-0310-a4f2-bffc1f526a37

Thanks, Ramiro Morales.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13608 bcc190cf-cafb-0310-a4f2-bffc1f526a37

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13180 bcc190cf-cafb-0310-a4f2-bffc1f526a37