Abort bps submission if submit-side run directory already exists.

@Arun Kannawadi was submitting a bunch of runs at the same time and was having problems (https://lsstc.slack.com/archives/C01FBUGM2CV/p1637273611121400)  The problems stemmed from the fact that due to a setting in the submit yaml all of the runs were using the same directory for their submit-side files.  A majority of the files are uniquely named across runs, but not all of them.  

We could go change the files we know are not uniquely named in both the bps core and the htcondor plugin.  However, this doesn't mean all WMS plugins will always name all of their files uniquely nor that there won't be a "bug" that introduces a non-unique filename.  Also, this can mean a ridiculous number of files ending up in same directories, complications debugging a single run, etc.

Until there is a specific use case that requires all in one directory or always guaranteeing unique filenames (without paths) for submit files, change bps to abort submission if submit directory already exists (try to reduce race conditions).