Improved error message display during validation errors.

When reporting a validation error (during model validation or fixture
loading, for example), the error messages now report the bad value as
well as the expected type. This can make identifying the offending field
and problem a bit easier.

Fixed #11595. Patch from raulcd and wildfire with supervision from
Russell.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16638 bcc190cf-cafb-0310-a4f2-bffc1f526a37