HTTP callback fixes

- Added HTTPError catching around the callback request, to not mark the
  entry as failed, just log the exception.
- Fixed bug where I forgot to actually fetch the entry before passing it
  to json_processing_callback.
- Changed __main__ migration #6 to create the ProcessingMetaData table
  as it is currently, to prevent possible breakage if a siteadmin
  is lagging behind with his db migrations and more than one migration
  wants to fix stuff with the ProcessingMetaData table.

mediagoblin/db/sql/migrations.py

@@ -106,5 +106,13 @@ def add_mediaentry_collected(db_conn):
 
 @RegisterMigration(6, MIGRATIONS)
 def create_processing_metadata_table(db):
-    ProcessingMetaData.__table__.create(db.bind)
+    metadata = MetaData(bind=db.bind)
+
+    metadata_table = Table('core__processing_metadata', metadata,
+            Column('id', Integer, primary_key=True),
+            Column('media_entry_id', Integer, ForeignKey(MediaEntry.id),
+                nullable=False, index=True),
+            Column('callback_url', Unicode))
+
+    metadata_table.create()
     db.commit()