Draft: In case of race condition in content_add, raise SerializationFailure instead of HashCollision.

Partially resolves #2019 (closed), by raising the appropriate exception.

This keeps the retry logic on the client side. I can add it on the server side with a decorator to both functions, but it won't be pretty because it needs to create a new cursor, so the test won't work as-is.

Migrated from D2248 (view on Phabricator)