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.
Having a look at processWebException() that you have removed, it is used in 3 places in the file and it does handle several error cases.  I wonder if the fix should make processWebException() handle the lack of headers instead of just not using it.