install-sh: a slightly better diagnostic, and tests enhancements

install-sh: a slightly better diagnostic, and tests enhancements

* lib/install-sh: When called with no non-option arguments and the '-t'
option with an argument that is not an existing directory, have the
diagnostic output complain about the lack of required arguments rather
than about the bad argument passed to '-t'.
* t/install-sh-unittests.sh: Enhance to also check diagnostic printed
in cases of expected failure.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>