Consider changing base class of CommunicationException to SystemException

CommunicationException derives from SystemException on .NET Framework. This was only introduced into netstandard2.0 so we had to switch to deriving from Exception. This has the consequence that the HResult value is different for CommunicationException and derived types between .NET Core and .NET Framework. On Framework, the HResult is COR_E_EXCEPTION (-2146233088), on Core, the HResult is COR_E_SYSTEM (-2146233087). This causes a difference in how FaultException is serialized.
We need to decide if we want to switch CommunicationException back to being derived from SystemException now it's available and discuss the compat implications.