Log information about missing target channel
Log information about missing target channel

E.g. when connecting via Mumble URL and the target channel does not exist

Fixes #2577

Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble,Lartza/mumble