t8003: check exit code of command and error message separately

t8003: check exit code of command and error message separately

Shell reports exit status only from the most downstream command
in a pipeline.  In these tests, we want to make sure that the
command fails in a controlled way, and produces a correct error
message.

This issue was known by Jay who submitted the patch, and also was
pointed out by Hannes during the review process, but I forgot to
fix it up before applying.  Sorry about that.

Signed-off-by: Junio C Hamano <gitster@pobox.com>