Fix SOAP fault parsing

Currently, when an HTTP 500 is returned, an exception is raised about a missing SOAP header. This is because SOAP faults from EWS don't contain headers. Instead of trying to parse the SOAP message the web existing exception handling logic should be called here.

I also cleaned up the `ServiceRequestBase` related classes a little bit, before the fix could be done.
Shouldn't there be a null check here before calling close() on the response.  Kind of similar to the what we do in the catch block above?