Catch error when bot user is not in private channel

[ddbruce]

Currently, no response is provided when /channel is used in a private channel and the @at-channel bot user isn't in the channel. This means that a user loses the message they wrote and may not understand that they need to add the bot user.

Ephemeral messages, I believe, can be sent to users in private channels regardless of the bot being a member, but we could also send a message to the Slackbot chat we currently use for moderated message submission confirmation. Any such message should include the originally sent text and instructions on how to add the bot user to the private channel.

[ddbruce]

Also, if someone accidentally tags @at-channel because they were unaware that @channel doesn't exist, that should be handled better than currently (i.e. not at all)