Document ctx.app.emit
The only place I've seen any information about `ctx.app.emit` is one line in https://github.com/koajs/koa/wiki/Error-Handling, but it's not clear how or why to use it. What is the point of doing this when in a middleware you can just try/catch and if there were any errors you just set the appropriate HTTP code and response body as in that example. You can simply do any logging or other work you need to there right?