make, ninja: Let a failing postbuild stop all later postbuilds for the same target.

Previously, all postbuilds would always be run. The old behavior matches Xcode 3,
the new behavior matches Xcode 4.

BUG=chromium:175867
Review URL: https://codereview.chromium.org/12230023

git-svn-id: http://gyp.googlecode.com/svn/trunk@1576 78cadc50-ecff-11dd-a971-7dbc132099af