Add `omim-directory` console entrypoint

[jsstevenson]

e.g.

$ disease-normalizer omim-directory
/User/jss009/.local/share/wags-tails/omim
$ mv mimTitles.txt $(disease-normalizer omim-directory)/omim_20240625.tsv

Or write it to support all sources (which would make sense, actually...)

Also double check how error description looks if file is missing