Fix #2751: javalib Files#delete now throws informative DirectoryNotEmptyException

[LeeTibbert]

For the 0.5.0 train This is the 0.5.0 version of PR #2754

The javalib Files#delete method now throws the DirectoryNotEmptyException when attempting
to delete a directory which is not empty. This Exception is a subclass of IOException.

The Java 8 documentation describes it as an "optional" specialization of IOException.
A more detailed Exception gives the end user a better clue.

This PR is the result of teamwork: I supplied the bugs & armanbilge supplied the
motivation and collaboration.

[armanbilge]

You did it!! Thank you so much!! Since it's okay to target to 0.4.x, would you mind changing the base branch in the PR so that we don't forget? Edit: also so that we run CI against the appropriate branch.

[LeeTibbert]

It is not pretty but it is, I believe, fit for purpose. Windows throws the same Exception, so there
is no off-putting quirk there.

re: Since it's okay to target to 0.4.x, would you mind changing the base branch in the PR so that we don't forget?
Is there git or GitHub magic to do that or do I have to start over, branch off of 0.4.5, copy files around, and
submit another PR? Will 0.4.n PRs get merged into the 0.5.0 stream, I do I need to maintain two PRs?

so that we run CI against the appropriate branch.
Now that would make too much sense. Does multi-arch work better in 0.4.n land?

[LeeTibbert]

This is ready for review. There is one of the usual failing multi-arch tests and
one failing Linux runtime test. The latter appears to have nothing to do with
this PR.