Return negative errno (instead of positive) on failure.

Return negative errno (instead of positive) on failure.

http://ag/489245 changed some return values from bools to errno values. However,
in forthcoming CLs, @lorenzo uses the convention of negative errno to indicate
failure. So, be consistent with that style.

Change-Id: I3eac8f142c36a2e779cda289c07ee374c49e2f6b