mgr/dashboard: front-end Grafana dashboard verification message is incorrect
The message says that the Grafana dashboard doesn't exist when the back-end returns anything other than "200", but that is not always correct. The same message is, for instance, also shown when the SSL configuration doesn't match the host or Grafana returns a 401, which makes it incorrect in those cases.
It would generally be more appropriate to have the message say that the dashboard couldn't be validated.
In addition to that, it might be helpful for the user to be able to display the response from Grafana for debugging purposes.