app: fix policy deny reporting when running in VM

qrexec-client-vm non-zero exit code means policy have denied the call.
Treat this exactly the same as empty response (in dom0 case).

qubesadmin/app.py

@@ -551,9 +551,8 @@ class QubesRemote(QubesBase):
             payload_stream.close()
         (stdout, stderr) = p.communicate(payload)
         if p.returncode != 0:
-            # TODO: use dedicated exception
-            raise qubesadmin.exc.QubesException('Service call error: %s',
-                stderr.decode())
+            raise qubesadmin.exc.QubesDaemonNoResponseError(
+                'Service call error: %s', stderr.decode())
 
         return self._parse_qubesd_response(stdout)