Handle the case of messages about users typing in non-existing channels
Handle the case of messages about users typing in non-existing channels

In some cases, not sure which, slack will tell us about users typing
in channels we don't know about.  Avoid generating a backtrace in that case.

rawdigits/wee-slack,scharfie/wee-slack,wee-slack/wee-slack,trygveaa/wee-slack,megalithic/wee-slack