Change handleError to throw or call next if not OAuthError

[chainlink]

If the error encountered is not expected (ie. not an OAuth Library error) it should re throw the error (or at least pass it to next to be dealt with)

https://github.com/oauthjs/express-oauth-server/blob/master/index.js#L142

I propose checking if it's an instance of https://github.com/oauthjs/node-oauth2-server/blob/master/lib/errors/oauth-error.js

If you agree I can prepare a PR

[maxtruxa]

None of the middlewares exported by OAuth2Server ever return an unknown error type. Unknown errors are always wrapped using ServerError (#authenticate, #authorize, #token).

With #19 merged you can initialize the server like this:

new OAuthServer({
  model: ...,
  useErrorHandler: true
});

which tells express-oauth-server to forward errors using next(err) instead of sending a response by itself.