Sync Gateway returns a 404 in the case where a deltaSrc revision was either not found
or was a tombstone (meaning we couldn't apply a delta on top of it), and the rev is considered a failed write and is not retried.
CBL in these cases return a 422 status code, to ask the sender to retry without deltas (CBG-881)
We should make ISGR return 422 status codes in the above cases to align with CBL and get the automatic rev retry handling. In addition, we may also want to consider retrying without deltas when we get 404, in order to allow for mixed-version ISGR compatibility.