If the destination domain doesn't exist anymore when the call gets
to qubesd, inform the client accordingly rather than giving it
a generic PermissionDenied error.
This enables client applications to handle such edge cases. Some
may want to inform the user about an incorrect request, others
(e.g. qvm-ls) may want to remove the specific domain from their
result list or try again later.
FixesQubesOS/qubes-issues#5105