spread: warn if MATCH exists with an unknown status

[mvo5]

We saw some obscure MATCH failures when running spread tests against snapd. It turns out the spread test is failing because of SIGPIPE which is triggered by us using "-o pipefail". To make it easier to find these kinds of issues print a warning in MATCH when an exit-code != 1 is returned (this is the exit code used when no match is found by grep).