saveKeysToDeviceObject is the detector of outgoing identity key errors.
Catch these key errors closer to the source by pulling the
getKeysForNumber into the context of sendMessageToDevices, which lets
it access registerError and the message protobuf.
Previously identity key errors would be uncaught if all existing
sessions with a recipient were closed/deleted, since we would
preemptively fetch the new identity key. The old error handling only
kicked in after a 409/410 response from the server when posting a
message encrypted for a stale session.
// FREEBIE
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