Handle another possible error in a unix pipe test

This cropped up on the bsd bot, and if it's an error that gets thrown then it's
fine to just whitelist another type of error in the test.
auto merge of #10133 : alexcrichton/rust/another-error, r=thestinger

This cropped up on the bsd bot, and if it's an error that gets thrown then it's
fine to just whitelist another type of error in the test.