[1.2.X] Fixed #13968 -- Fixed SelectDateWidget processing of an invalid date...

[1.2.X] Fixed #13968 -- Fixed SelectDateWidget processing of an invalid date input value when USE_L10N is on, for consistency with its behavior when USE_L10N=False (now the form field reports the validation error in both cases). Thanks mitar for the report.

Backport of [15416] from trunk

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15417 bcc190cf-cafb-0310-a4f2-bffc1f526a37