[1.2.X] Fixed #15167 -- Ensure that non-form errors are always part of an...

[1.2.X] Fixed #15167 -- Ensure that non-form errors are always part of an ErrorList. Thanks to Harm Geerts for the report and patch.

Backport of r15424 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15425 bcc190cf-cafb-0310-a4f2-bffc1f526a37