qvm-device: improve error message on invalid device type

Fixes QubesOS/qubes-issues#6057

qubesadmin/tools/qvm_device.py

@@ -81,6 +81,7 @@ def list_devices(args):
     app = args.app
 
     devices = set()
+    try:
         if hasattr(args, 'domains') and args.domains:
             for domain in args.domains:
                 for dev in domain.devices[args.devclass].attached():
@@ -92,6 +93,10 @@ def list_devices(args):
             for domain in app.domains:
                 for dev in domain.devices[args.devclass].available():
                     devices.add(dev)
+    except qubesadmin.exc.QubesDaemonAccessError:
+        raise qubesadmin.exc.QubesException(
+            "Failed to list '%s' devices, this device type either "
+            "does not exist or you do not have access to it.", args.devclass)
 
     result = {dev: Line(dev) for dev in devices}