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`.
Merge pull request #2384 from joshmoore/9785-list-scripts

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