Stop checking separately for ConnectionDone/ConnectionLost, since those have
been folded into DeadReferenceError since foolscap-0.3.1 . Write
rrefutil.trap_deadref() in terms of rrefutil.trap_and_discard() to improve
code coverage.
This facilitates client code to easily catch ServerFailures without also catching exceptions arising from client-side code.
See also:
http://foolscap.lothar.com/trac/ticket/105 # make it easy to distinguish server-side failures/exceptions from client-side