Check initialization of workers

[d-todt]

The runner should check if the workers were launched successfully. In the current version, L2L does not notice if the worker failed immediately, e.g. due to an illegal combination in the srun command. The user will end up with a confusing error message which does not reflect the real reason behind the failure.

This is how I checked the status of the workers:

process = subprocess.Popen(...) # starting srun in the runner
# ...
exitcode = process.poll()
if exitcode is not None: # None would imply that it's running
    logger.info(str(process.stdout.read()))
    logger.info(str(process.stderr.read()))