[1.2.X] Fixed #13610 -- Improved error reporting when the ENGINE setting is...

[1.2.X] Fixed #13610 -- Improved error reporting when the ENGINE setting is ommitted from a database configuration. Thanks to Gregor Müllegger for the report and patch.

Backport of r13489 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13490 bcc190cf-cafb-0310-a4f2-bffc1f526a37