run-dart-action.py improvements

* Let Ctrl-C stop the script by making worker threads daemon threads.
* If a script is missing log an error. Previously a missing script
  would cause the program to never terminate.
* Add -j/--jobs to control the number of concurrent jobs.
* Don't show the output of successful scripts unless -v/--verbose is
  supplied.
* Show progress.

Change-Id: I780de1618e8427748ee962dbedbfc26be6300b86
1 file changed