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Improve logging for unprocessable entity 422 issue because GitHub does not return an informative HTTPError. 422 error is caused by (1) branch already exists but somehow create_pr is reached (2) missing Change_Id in the original CL.

Bug: 1056378
Change-Id: I92e0c59f30c5436121ac88af3972a3be0e2a46bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097156
Commit-Queue: Kyle Ju <kyleju@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750619}