too errors if not in subdirectory of HOME

[mahiki]

Test the input path and exit with an error message if the cwd is not subdir of HOME.

too big
Traceback (most recent call last):
  File "/opt/homebrew/bin/too", line 8, in <module>
    sys.exit(main())
  File "/opt/homebrew/Cellar/desertislandutils/0.1.0/libexec/lib/python3.9/site-packages/src/toobigdatadoc/too.py", line 85, in main
    make_topdir_and_link(too_rel_path(Path.cwd()))
  File "/opt/homebrew/Cellar/desertislandutils/0.1.0/libexec/lib/python3.9/site-packages/src/toobigdatadoc/too.py", line 77, in too_rel_path
    path_from_home = any_path.relative_to(Path.home())
  File "/opt/homebrew/Cellar/[email protected]/3.9.10/Frameworks/Python.framework/Versions/3.9/lib/python3.9/pathlib.py", line 939, in relative_to
    raise ValueError("{!r} is not in the subpath of {!r}"
ValueError: '/private/tmp/billlll' is not in the subpath of '/Users/segovia' OR one path is relative and the other is absolute.