Properly handle errors in GRIT in the scons build.
Properly handle errors in GRIT in the scons build.

This lets the child process exit even if there are errors and causes
the main scons process to identify these errors as build failures.

Review URL: http://codereview.chromium.org/28282


git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@10652 0039d316-1c4b-4281-b951-d872f2087c98

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