Improved diagnostics for being unable to find the cosmic-ray executable

[rob-smallshire]

I'm running into this while distributing cosmic-ray workers in the cloud. I know how to fix it, it's here as a reminder.

$ cosmic-ray exec celery-session-2.json 
Traceback (most recent call last):
  File "/Users/rjs/.virtualenvs/asm68/bin/cosmic-ray", line 11, in <module>
    load_entry_point('cosmic-ray', 'console_scripts', 'cosmic-ray')()
  File "/Users/rjs/dev/cosmic-ray/cosmic_ray/cli.py", line 363, in main
    exit_at_end=False)
  File "/Users/rjs/.virtualenvs/asm68/lib/python3.6/site-packages/docopt_subcommands/__init__.py", line 69, in main
    result = commands(argv)
  File "/Users/rjs/.virtualenvs/asm68/lib/python3.6/site-packages/docopt_subcommands/subcommands.py", line 147, in __call__
    return handler(config)
  File "/Users/rjs/dev/cosmic-ray/cosmic_ray/cli.py", line 178, in handle_exec
    cosmic_ray.commands.execute(session_file)
  File "/Users/rjs/dev/cosmic-ray/cosmic_ray/progress.py", line 97, in wrapper
    return func(*args, **kwargs)
  File "/Users/rjs/dev/cosmic-ray/cosmic_ray/commands/execute.py", line 47, in execute
    for work_item in work_items:
  File "/Users/rjs/dev/cosmic-ray/plugins/execution-engines/celery3/cosmic_ray_celery3_engine/engine.py", line 22, in __call__
    yield WorkItem(result.get())
  File "/Users/rjs/.virtualenvs/asm68/lib/python3.6/site-packages/celery/result.py", line 175, in get
    raise meta['result']
celery.backends.base.FileNotFoundError: [Errno 2] No such file or directory: 'cosmic-ray': 'cosmic-ray'