Add QubesValueError to mock qubesadmin.exc

Will be needed in upcoming changes, to make pylint happy.

test-packages/qubesadmin/exc.py

@@ -36,3 +36,6 @@ class BackupAlreadyRunningError(BaseException):
 
 class QubesDaemonCommunicationError(BaseException):
     pass
+
+class QubesValueError(BaseException):
+    pass