When the test client detects a redirect to a URL seen in the
currently followed chain it will now raise a RedirectCycleError
instead of just returning the first repeated response.

It will also complain when a single chain of redirects is longer
than 20, as this often means a redirect loop with varying URLs,
and even if it's not actually one, such long chains are likely
to be treated as loops by browsers.

Thanks Preston Timmons, Berker Peksag, and Tim Graham for reviews.

This is a security fix. Disclosure following shortly.

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