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.
I'm not sure we should wrap the exception here either. The problem is the exception gets wrapped two or three times by this class and the problem with wrapping exceptions is that it makes handling the real exception much harder. Why are exceptions wrapped anyway?