List subcommand fails if jobs get cleaned manually

[camall3n]

If any .onager/scripts/jobname directory gets removed, onager list fails completely instead of simply listing the jobs it can find.

$ onager list
Traceback (most recent call last):
  File "./venv/bin/onager", line 18, in <module>
    list_.list_commands(args)
  File "./venv/lib/python3.7/site-packages/onager/list.py", line 53, in list_commands
    job_list = get_job_listings(args)
  File "./venv/lib/python3.7/site-packages/onager/list.py", line 36, in get_job_listings
    commands, tags = load_jobfile(jobfile)
  File "./venv/lib/python3.7/site-packages/onager/utils.py", line 8, in load_jobfile
    with open(jobfile_path, 'r') as file:
FileNotFoundError: [Errno 2] No such file or directory: '.onager/scripts/[jobname]/jobs.json'