Process: add method to test for failure based on exitcode

[stdweird]

most code does a simple if($?) after CAF:Process; but there is code (like ncm-download) that uses more leborate check

...
$proc->execute();
if (!POSIX::WIFEXITED($?) || POSIX::WEXITSTATUS($?) != 0) {
    $self->error("could not get GSSAPI credentials: $errs");                              
    return 0;
}

figure out what the best way is to check for success (maybe redefine $? based on above checks), and also add it as a new method.