Move InvalidId to db.util

Instead of import InvalidId from the low level bson module,
first import it in our db.util wrapper and second import it
from pymongo.errors.

mediagoblin/db/util.py

@@ -17,4 +17,5 @@
 
 # Imports that other modules might use
 from pymongo import DESCENDING
+from pymongo.errors import InvalidId
 from mongokit import ObjectId

mediagoblin/decorators.py

@@ -15,11 +15,10 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 
-from bson.errors import InvalidId
 from webob import exc
 
 from mediagoblin.util import redirect
-from mediagoblin.db.util import ObjectId
+from mediagoblin.db.util import ObjectId, InvalidId
 
 
 def _make_safe(decorator, original):