Convert C++HL exception test to Gmock, and fix a bug in the exception converter.

* subversion/bindings/cxxhl/src/exception.cpp (detail::checked_call):
   Use separate ErrorBuilder and CancelledBuilder internal classes in order
    to avoid the vagaries of slicing constructors.

* subversion/bindings/cxxhl/tests/test_exception.cpp
  (traceall, tracecheck, DummyTest): Removed.
  (CatchError): Replaces test_error, converted to Gmock.
  (CatchCancelled): Replaces test_cancelled, converted to Gmock.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507987 13f79535-47bb-0310-9956-ffa450edef68