"Not enough datasets" should be 503, not 500?

[davepacheco]

While writing some new authz tests for "disk create" (before I'd added authz protection for that endpoint), I ran into this error:

[2022-01-31T11:07:40.579360879-08:00]  INFO: d777d6f2-bbd2-4e1a-9ac2-e862d35da12d/dropshot_external/146 on ivanova: request completed (req_id=22e6b82f-d21d-4faf-9d80-30da6d6fead1, method=POST, remote_addr=127.0.0.1:63017, local_addr=127.0.0.1:64252, error_message_external="Internal Server Error", response_code=500)
    uri: /organizations/demo-org/projects/demo-project/disks
    --
    error_message_internal: Transaction error: Custom transaction error; Not enough datasets for replicated allocation: 0

I believe this should be a 503 error and not a 500. I think of 500s as bugs, while "insufficient capacity right now" is the same as "unavailable right now", which would be a 503.