Explicitly prevent the bot from connecting to two servers' voice channels at the same time

[svioletg]

It's currently advised in documentation to not use the same instance of bot for more than one voice channel at a time in different servers, as doing this causes issues that for the time being I am not able to sort out. It would be better to have the bot actually detect if this has been attempted, and prevent it if so. A config option can be created so that this is left optional.

[svioletg]

Doesn't reference the server, but a check to see if the channel is the same has been put in, so this will work for checking servers implicitly. Although, it may be good to have a command to move the bot to a new channel in the future.