Tidy up marshal exceptions

[bernardnormier]

The following marshaling exceptions are thrown by the run time, but never caught:

• ProxyUnmarshalException
• StringConversionException
• EncapsulationException

I don't see any conceivable reason for why an application would want to catch these exceptions specifically, seeing that the all relate to marshaling errors that indicate that a request was internally inconsistent.

I'd like to replace these with MarshalException, with appropriate message strings. At the moment, we are carrying the generated code for these exceptions around for no good reason.