Ignore files already deleted on GCS file deletions

Relying on the GET request issued first to fetch the file we want to
delete is not enough to avoid this error. If the file is deleted after
our GET request but before the DELETE request we'll get a NotFound error
that after all means that the file is already deleted, so it can be
safely ignored.

activestorage/lib/active_storage/service/gcs_service.rb

@@ -35,7 +35,11 @@ def download(key)
 
     def delete(key)
       instrument :delete, key do
-        file_for(key).try(:delete)
+        begin
+          file_for(key).try(:delete)
+        rescue Google::Cloud::NotFoundError
+          # Ignore files already deleted
+        end
       end
     end