stream error

we only do `try catch` here to catch the error.
but all the streams' and events' errors are handling by `stream.on('error', this.onerror)`, we can do nothing for them, except hack `ctx.onerror` method.

should we add an easy way for users to write their own error handler in koa, just like what connect/express do:

```
app.use(function (err, req, res, next){});
```

or just leave these to error middleware with hack `ctx.onerror`?

Start by tracing the Koa error middleware and the stream.on('error', this.onerror) path mentioned in the issue, including how ctx.onerror is currently used. The issue names no files or tests, so first identify the relevant entry points and existing error-handling coverage. Done requires a decided, documented approach for user-defined handling of stream and event errors.

Written by the indexing model from the issue text.