Handle private channels/improve channel info error handling and logging

[mbland]

While figuring out why https://github.com/mbland/hubot-slack-github-issues wasn't working for @geekdave, after some back-and-forth on 18F#51, we figured out it was because he was trying to use it in a private Slack channel.

What happened was that the channel ID was not in the Slack RTM client's channel cache, so SlackRtmDataStore.channelById was returning null. Impl.processMessage from lib/reaction-issue-filer.js wasn't expecting or handling this, hence the error.

After reviewing https://api.slack.com/rtm#channel_selection and https://api.slack.com/methods/rtm.start, it appears the solution may be a matter of trying to look in the groups and mpims caches for incoming channel IDs starting with G and the ims cache for IDs starting with D. (Note that, per the Slack rtm.start doc, "MPIMs are only returned to the client if mpim_aware is set when calling rtm.start.")

Reproducible in:

hubot-slack-github-issues version: git+https://git@github.com/mbland/hubot-slack-github-issues.git
OS version(s): any
Device(s): any

Steps to reproduce:

1. Add git+https://git@github.com/mbland/hubot-slack-github-issues.git to your Hubot instance.
2. Per the https://github.com/mbland/hubot-slack-github-issues README, add config/slack-github-issues.json to your Hubot instance. Use the default all-channel emoji from the mbland/hubot-slack-github-issues repo or configure your own.
3. Set the following environment variables:

   HUBOT_SLACK_TOKEN="<your Hubot's Slack API token>"
   HUBOT_GITHUB_TOKEN="<a GitHub token for your Hubot user>"
   HUBOT_LOG_LEVEL="debug"

4. Launch your Hubot instance.
5. In Slack, post a message to a private channel/group or direct message session.
6. Add the emoji configured in step 1 to the message.

Expected result:

As with normal Slack channels, the plugin should react to the reaction_added event, file a GitHub issue, and respond to the user who added the reaction with the URL of the newly-created issue.

Actual result:

Hubot reacts to the event, but responds to the user with Cannot read property 'name' of undefined. The logs show a listener executed for Message [Object object] followed by DEBUG messages containing Sending to G... followed by Cannot read property 'name' of undefined. See @geekdave's screenshot from 18F#51, also attached below, for a concrete example.

Note that I'm working separately on making the error handling and logging more robust, regardless of upstream changes to hubot-slack.

Attachments:

Actions:

• guard against the channel info coming back as undefined to the extent possible, i.e. figure out how to handle messages from private channels if possible
• produce a better error response if it still does come back undefined