webnut crashing with `internal server error` when receiving an `ERR` message from the nut-server

[gitmachtl]

@TechaNima reached out to me with a problem using webNUT in combination with my apcupsd NUT wrapper script i wrote a while ago.

gitmachtl/various#5 (comment)

webNUT is requesting descriptions from the NUT-Server. if the server responds with a valid ERR UNKNOW-COMMAND, webNUT crashes with an internal server error.

meanwhile, i have added the GET UPSDESC <upsname> command and also the GET DESC <upsname> <varname> command.
but webNUT should not crash at any point when receiving an error message ERR ... from a NUT-Server.

best regards,
martin