buildbot: Early return to ensure failures appear at end of build step.
buildbot: Early return to ensure failures appear at end of build step.

When building multiple architectures in the same step, early
return on first failure, so the failure always shows up at
the end of a given step.

R=bradnelson@google.com

Review URL: https://codereview.chromium.org/335123007

git-svn-id: 84b3587ce119b778414dcb50e18b7479aacf4f1d@1338 7dad1e8b-422e-d2af-fbf5-8013b78bd812

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