Fix media display for no media_data row in sql.

If there is no media_data row for the current media (for
whatever reason, there might be good ones), let
MediaEntry.media_data not raise an exception but just
return None.

The exif display part now handles this by checking whether
.media_data.exif is defined (None has no attribute exif, so
it's undefined, all fine).

mediagoblin/db/sql/models.py

@@ -176,7 +176,7 @@ class MediaEntry(Base, MediaEntryMixin):
         session = Session()
 
         return session.query(self.media_data_table).filter_by(
-            media_entry=self.id).one()
+            media_entry=self.id).first()
 
     def media_data_init(self, **kwargs):
         """