Better handling of ACCESS-DENIED errors (Synology DSM)

[gbakeman]

See #79 and #80

Synology DSM software includes a modified version of the NUT upsd server in order to interface with their web frontend, in particular to utilize a client whitelist for any connections to the server. In some cases (see the issues above) this has resulted in some unexpected behavior, but more importantly users connecting to Synology devices tend to see the ACCESS-DENIED error more often than anyone else. It would be worth having a user-friendly UI to handle the access denied errors, even something as simple as some functionality to detect the DSM upsd software and prompt the user to whitelist their client in the web UI.