#8 task-executor: handle script failures

[Daniel Kigelman]

When the task-executor is attempting to execute a script (e.g. interactive_pwscf.sh) and the espresso executable referenced in the script does not run (e.g. file not found), the task-executor sleeps for a long time. I don't know if it ever exits by itself.

Several ways to address this:

1. limit amount of sleep

2. redirect the standard error to a file, and check the contents of that file for errors

3. perhaps there is a way to check if a program succeeded (something like ./bin/startup.sh && tail -f logs/catalina.out <-- the second command does not execute if the first one fails)