Improved error message if DATABASE_ENGINE is invalid.

    backend_dir = os.path.join(__path__[0], 'backends')

    available_backends = [f for f in os.listdir(backend_dir) if not f.startswith('_') and not f.startswith('.') and not f.endswith('.py') and not f.endswith('.pyc')]

    available_backends.sort()

    raise ImproperlyConfigured, "Could not load database backend: %s. Is your DATABASE_ENGINE setting (currently, %r) spelled correctly? Available options are: %s" % \

        (e, settings.DATABASE_ENGINE, ", ".join(map(repr, available_backends)))

    if settings.DATABASE_ENGINE not in available_backends:

        raise ImproperlyConfigured, "%r isn't an available database backend. vailable options are: %s" % \

            (settings.DATABASE_ENGINE, ", ".join(map(repr, available_backends)))

    else:

        raise # If there's some other error, this must be an error in Django itself.

get_introspection_module = lambda: __import__('django.db.backends.%s.introspection' % settings.DATABASE_ENGINE, '', '', [''])

get_creation_module = lambda: __import__('django.db.backends.%s.creation' % settings.DATABASE_ENGINE, '', '', [''])