contrib: Fix exit status of relpath.sh for invalid args [PR125956]

The relpath.sh script always writes its usage message to stdout and
exits with zero status, even for invalid arguments. This caused a
problem for libstdc++-v3/src/c++23/Makefile which assumed that
relpath.sh won't exit successfully on error, via:

relpath=$(relpath.sh ...) && sed 's,@MODPATH@,$$relpath,' ...

Because relpath.sh exits successfully for invalid args, the sed command
still ran even when relpath.sh was invoked incorrectly.

The libstdc++ Makefile has been changed to work around this, but
relpath.sh should still have an idiomatic exit status.

contrib/ChangeLog:

	PR libstdc++/125956
	* relpath.sh: For invalid arguments write to stderr and exit
	with non-zero status.