handle FileNotFoundError in --outputDir argument

[arezaii]

It's possible to encounter an uncaught FileNotFoundError if you specify a path to the --outputDir argument with any part of the parent directory structure not yet existing on disk.

Output directory does not exist, creating: (full path)/frontier-2-node-single-HPL-run/outputs
uncaught FileNotFoundError: No such file or directory (in mkdir with path "(full path)/frontier-2-node-single-HPL-run/outputs")

If I had created frontier-2-node-single-HPL-run then there wouldn't be a problem.

This seems like a common error a user could encounter and so handling it makes sense to me for 2 reasons:

1. we can tailor the error message to the scenario
2. it makes it look like an expected exception in this case