Attempting to use the create or modify user account admin api to add an external ID to a user that is already mapped to another user fails with a unique constraint db error. It should really return a HTTP 400 error instead.

In addition, any existing external IDs for the user that we were trying to modify will be erased (all entries for that user are removed from the user_external_ids table. This is due to the fact that the removal and addition of old and new IDs are separate actions:

They should really be bundled into a single database transaction in a new storage function so that it's not possible for the user to be left without any mappings.