Throw errors when passthroughErrors is enabled

When passing through errors, one expects koa-oauth-server to throw in order to handle the error in a separate  handler. This is useful when you want to compose middleware, for example.

Previously, passing through errors would only emit an error on the app's event emitter.
I think just checking 500 might be a bit general? What about if we put:
```js
app.use(function *(next) {
  try {
    yield next;
  } catch {
    this.body = 'Caught OAuth exception';
  }
});
```

That way we're checking an exception has been thrown? What do you think?