Return no results from grep repository queries on error

Summary: Fixes T7852. Although 1 could also indicate other kinds of problems, assume it means "no results".

Test Plan: Searched for nonsense strings in Git and Mercurial. Searched for valid strings in Git and Mercurial.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7852

Differential Revision: https://secure.phabricator.com/D14943