cpp: Return actual error if revocation fails

* lang/cpp/src/editinteractor.cpp, lang/cpp/src/editinteractor.h
(EditInteractor::parseStatusError): New.
* lang/cpp/src/gpgrevokekeyeditinteractor.cpp
(GpgRevokeKeyEditInteractor::Private::nextState): Handle status
error.

With this change the interactor returns a proper error like "Bad
Passphrase" or "Empty Passphrase" instead of an unspecific "General
Error" if a status error occurred.

• GnuPG-bug-id: T5904