duckpan: fail on missing include directory.

Using realpath instead of user-entered string, in order to possibly help
them spot the problem.

Addresses #156.
bin/duckpan: give them the absolute path, if we can't get realpath

I still think the realpath is more useful, but if throws an exception,
just go with absolute.

The absolute is not in a `catch` block, because we lose our `$_`.  If
this was any more complex I would write it a different way.

Also, split out into to sub-tests.
Merge pull request #157 from mwmiller/inclusiveness

duckpan: fail on missing include directory.