Improve error handling and presentation

[xclydes]

These changes address the scenarios where:-

1. The request throws and IOException due to a read timeout. i.e. The response from the server took 20 seconds (the default timeout).
2. The server response with a non 2xx header and the request throws a HttpResponseException. The exception thrown is an extension of the IOException class and results in the details getting lost.

The changes to OAuthClient.java allows a custom initializer to be set by external classes, which in turn can specify a custom timeout.

The changes toUpworkRestClient.java are to change the non 2xx behaviour on the request and correct a cast error that was thrown whenever genError(HttpResponse response) was called.

[sageerify]

wow