Get a traceback when running `opensafely` in a deleted directory on Linux

Reproduce:

1. Enter a directory in your terminal.
2. Delete that directory.
3. In the same terminal as 1, run opensafely --version

Traceback (most recent call last):
  File "…bin/opensafely", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "…opensafely/__init__.py", line 90, in main
    add_subcommand("jupyter", jupyter)
  File "…opensafely/__init__.py", line 83, in add_subcommand
    module.add_arguments(subparser)
  File "…opensafely/jupyter.py", line 38, in add_arguments
    default=os.getcwd(),
            ^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory

This could be a nicer error.

Start with opensafely/init.py and opensafely/jupyter.py, especially the add_arguments call and the os.getcwd() default shown in the traceback. Reproduce the command from a deleted directory, then verify that opensafely --version no longer emits a traceback and instead gives a user-facing result.