When a DNS query cant be sent because it is invalid, notify the user via the error log

[woodsaj]

Currently all errors making DNS requests are treated the same, whether they are due to errors preventing a request being sent or errors with the response received.

Before trying to send a request to all configured servers, we need to first check that the request is valid and if not, set the reason in the error log.

[Dieterbe]

would it be possible to:

1. provide direct feedback to user at the monitor configuration stage, that their request is invalid?
2. block them from saving invalid requests