Remove intentionally-breaking "data" symlink and add better detection

This allows for a more graceful error message for the case of users who need to update their volume path.

I tested this with 17 and `--env PGDATA=/var/lib/postgresql/17/docker` (to make sure it still works fine and doesn't complain about the extra volume provided by Docker/`VOLUME`) and 18 with `-v /var/lib/postgresql/data` (where it appropriately errors and explains to me what I should be doing instead).

I also did a reflow on some text in the error message that's designed to fit in 80 columns but was taking 81. 😂

Author: tianon