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.
Well, the exception handling/wrapping in this library is kind of an issue on its own. Every method seems to just throw generic Exceptions all over the place, instead of throwing specific exceptions which can be handled adequately. If you think I should wrap it here already, I'll make the change, but I'm unsure here.