Option to not add the bot to a team, and proper error handling if the bot doesn't belong to a team.

[a-gerhard]

We run a chat bot that only messages users. There is no need for the bot to be in a team on our mattermost server.

• If the bot is not in the configured team (and not a mattermost admin), this line https://github.com/errbotio/err-backend-mattermost/blob/master/mattermost.py#L346 will throw an error. You should add a proper error message what is wrong. Removing this one line allowed our bot to run perfectly until I took a bit deeper into the issue.
• You should add an option for the bot to not belong to a team, e.g., by leaving the team empty.