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.