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>