Improve error message on missing lib/scripts (Fix #9785)

Now when lib/scripts is deleted or removed, a more appropriate
error message is thrown. The server, however, won't start up
in this condition, and any attempt to read the directory results
in an `InternalException`.

To test:
 * build normally and start up the server
 * `mv dist/lib/scripts dist/lib/scripts.tmp`
 * list the scripts (`dist/bin/omero script list`)
 * Moving the directory back should return things to normal.