Don't raise an error on duplicate placements

This can happen if two pushes are racing on the same storage.

data/model/storage.py

@@ -39,7 +39,10 @@ def get_image_location_for_id(location_id):
 def add_storage_placement(storage, location_name):
   """ Adds a storage placement for the given storage at the given location. """
   location = get_image_location_for_name(location_name)
-  ImageStoragePlacement.create(location=location.id, storage=storage)
+  try:
+    ImageStoragePlacement.create(location=location.id, storage=storage)
+  except IntegrityError:
+    # Placement already exists. Nothing to do.
 
 
 def garbage_collect_storage(storage_id_whitelist):

workers/storagereplication.py

@@ -46,7 +46,6 @@ class StorageReplicationWorker(QueueWorker):
       return True
 
     # For any missing storage locations, initiate a copy.
-    storage_paths = StoragePaths()
     existing_location = list(partial_storage.locations)[0]
 
     for location in locations_missing: