lowlevel-blt-bench: make extra arguments an error

If a user gives multiple patterns or extra arguments, only the last one
was used as the pattern while the former were just ignored. This is a
user error silently converted to something possibly unexpected.

In presence of extra arguments, complain and quit.

Cc: Ben Avison <bavison@riscosopen.org>
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>