cmWorkerPool: Show process exit code / signal number upon failure
Previously if an AUTOGEN process like moc exited with a non-zero exit code, cmWorkerPool would not populate the result's ErrorMessage because it mistakenly thought that if the ExitCode / TermSignal had a non-zero value, the ErrorMessage must have already been set.
This was a wrong assumption.
Instead, check if ErrorMessage is empty, and if it is, set it to whatever the exit code or term signal number is.
Backport: release
Edited by Brad King