Fix error message when using "gcl try nonexistentcl".
Fix error message when using "gcl try nonexistentcl".

Call ChangeInfo.Load() with fail_on_not_found=True to get a good error message in this case.

BUG=none
TEST=manual

Review URL: http://codereview.chromium.org/8692007

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@112018 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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