Fixed #8904 -- Improved error message when spatial lookup is not available, and changed exception to `ValueError` to prevent swallowing of error on queryset evaluation.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12887 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/contrib/gis/db/models/fields.py

@@ -241,7 +241,8 @@ class GeometryField(Field):
 
             return params
         else:
-            raise TypeError("Field has invalid lookup: %s" % lookup_type)
+            raise ValueError('%s is not a valid spatial lookup for %s.' %
+                             (lookup_type, self.__class__.__name__))
 
     def get_prep_lookup(self, lookup_type, value):
         if lookup_type == 'isnull':