Not all files in the submit-side directory are uniquely named.  Hence
using the same submit directory for multiple runs will cause problems.
Modify BPS to abort a submission if the run directory already exist.

The default error message from Python subprocess.run() when creating the
execution Butler failed was not very informative.  Customized it to
point the user to the execution Butler log file where more useful
information about the cause of the problem may be found.