Changed _on_close method tornado/simple_httpclient.py to raise the error instead of raising an HTTPError

This is a patch for https://github.com/facebook/tornado/issues/1033

1. Changed tornado/simple_httpclient.py to raise the error instead of storing the error message in a string and then raising an HTTPError. 
2. Changed the websocket tests for network_fail and network_timeout to expect an IOError instead of an HTTPError. No error numbers are checked as these are platform independent. It should be sufficient to check the error types.