Whenever unhandled errors are raised, it's especially important for us
to get some hints of what happened. In a perfect world these kinds of
errors shouldn't happen, but ofcourse they do for time to time..

The last time we had this kinds of error, there were no errors visible
in the logs, other than seeing that the bot process seemed to restart
more than usual.

While working in these changes, I found that the default unhandled
error handler in express.js, writes the error to stdout, not into the
log log files we've write through via bunyan.

With these changes in place we'll get something like this in the logs:

```
19:16:26.303Z FATAL bot: Unchaught exception, terminating bot process immediately
    ReferenceError: invokingNoneExistentFunction is not defined
        at Promise.resolve.then (/github-bot/app.js:47:31)
        at process._tickCallback (internal/process/next_tick.js:109:7)
        at Module.runMain (module.js:613:11)
        at run (bootstrap_node.js:387:7)
        at startup (bootstrap_node.js:153:9)
        at bootstrap_node.js:500:3
```

Refs nodejs#174