Add error when path to cmssw release in config isn't correct

[hannahbnelson]

Currently, if the path to the cmssw release isn't correctly specified in the lobster config file, lobster process config.py will run without giving any errors, and it will seem like the job submitted correctly. However, without the cmssw release the jobs can't run so when a work_queue_factory is made, there aren't any jobs to pick up.

There should be an error added that alerts the user that the cmssw release path in the config isn't correct.