There are cases when destination domain doesn't exist when the call gets
to qubesd. Namely:
1. The call comes from dom0, which bypasses qrexec policy
2. Domain was removed between checking the policy and here
Handle the the same way as if the domain wouldn't exist at policy
evaluation stage either - i.e. refuse the call.
On the client side it doesn't change much, but on the server call it
avoids ugly, useless tracebacks in system journal.
FixesQubesOS/qubes-issues#5105
eb39f69882