fix(console): a serial console whose VM was deleted meanwhile says so

consolecallback read the VM's record with .get(): one deleted between
socketiod's accept and the worker's start ended with a Python traceback
in the user's terminal, which is the worker's stderr. It now says the VM
is no longer in the panel.

conf/daemon/consolecallback

@@ -115,7 +115,10 @@ def main():
     if os.environ.get("CONSOLECALLBACK_LOG_FD"):
         log.addHandler(logging.StreamHandler(os.fdopen(int(os.environ["CONSOLECALLBACK_LOG_FD"]), "w", buffering=1)))
         log.propagate = False
-    instance = Instance.objects.select_related("compute").get(pk=int(sys.argv[1]))
+    instance = Instance.objects.select_related("compute").filter(pk=int(sys.argv[1])).first()
+    if instance is None:  # deleted since socketiod accepted the token
+        say("The VM is no longer in the panel")
+        sys.exit(1)
     user_id = int(sys.argv[2])
     c = instance.compute
     try:

console/tests/test_consolecallback.py

@@ -106,3 +106,10 @@ class AccessTests(DjangoTestCase):
             ownership.delete()
             self.assertFalse(cb.may_still_open(owner.pk, vm))
             self.assertFalse(cb.may_still_open(999999, vm))
+
+    def test_a_vm_deleted_since_the_connect_is_told_not_a_traceback(self):
+        cb = load()
+        with patch.object(cb.sys, "argv", ["consolecallback", "999999", "1"]), patch.object(cb, "say") as say, \
+                self.assertRaises(SystemExit):
+            cb.main()
+        say.assert_called_once_with("The VM is no longer in the panel")