Read only stdout in LinesOfALargeExecFuture

Summary:
We read all the output before throwing so stderr is empty.
I hit this issue in D5738.

Test Plan: Searched for invalid regexp in D5738.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5767