Handle when a patchset doesn't exist instead of printing a stack trace.
Handle when a patchset doesn't exist instead of printing a stack trace.

Print a meaningful error message so the user can try diagnosing himself.

R=rogerta@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/10913026

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@154436 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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