exits successfully($? is 0) when it fails to create notes

[rmNULL]

Issue Summary

When $NOTES_DIRECTORY is set to a filename, that already exists in the directory.
It opens a new note but fails to save it, and exits 0
(as the exit status depends on the editor)

Steps to Reproduce

1. touch /tmp/secret
2. env NOTES_DIRECTORY=/tmp/secret notes n leaks
3. edit and save(fails to save), quit forcefully.
4. Once you are back at shell, echo $?

Expected Behavior

exit before opening the editor and ask user to set NOTES_DIRECTORY to a safe location.
ofc, exit with non-zero status

[pimterry]

So to be clear, you'd like new & open to check in advance whether NOTES_DIRECTORY is actually a directory, right? Sounds sensible to me if so, I'd accept a PR.

[rmNULL]

I feel it would be better if we check after reading the configuration file.