Fixed #12467 -- Made the model data validation for `DateField` and...

Fixed #12467 -- Made the model data validation for `DateField` and `DateTimeField` more useful by actually telling what was the value that failed. Also did a bit of PEP8 cleanup in the area. Thanks to knutin for the report, to raulcd for the initial patch and to charettes for the review.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16966 bcc190cf-cafb-0310-a4f2-bffc1f526a37