Sending messages to channels without requisite permissions result in a restart

[williammck]

We don't currently have checks in place for whether we're able to send messages to a given channel.
Because of this, if a channel we're observing doesn't grant us send permissions, the bot will crash out and restart.
We should ideally check this before even generating a message, as otherwise we're spending useless cycles when another channel could instead get a message.

A/C: Stop generating messages for channels we can't send in