Improve error message for permission errors.

On #565 (closed), I suggested that we expand this message:

Incorrect permissions: / is u=rwx,g=rwx,o=rwx; must be g-w,o-w

With a helpful message something like:

Permissions are set too permissively on the directory /: that's dangerous, since other users
could modify that directory's contents and override our behavior! You can fix this by making
more of your filesystem read-only. You can suppress this message by setting storage.permissions.dangerously_trust_everyone=true, or setting 
ARTI_FS_DISABLE_PERMISSION_CHECKS=yes in your environment.

See also #579 (closed), where I ask how we should go about adding a message that long.