Give better error messages when changes aren't mergeable.

Instead of reporting that a change is "not commit ready
anymore", we should give a specific error message explaining what the
exact problem is.

BUG=chromium:336632
TEST=Updated unit tests.

Change-Id: Ibd852afab6d2603dd9a068b99145d88d1af92068
Reviewed-on: https://chromium-review.googlesource.com/235790
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Tested-by: David James <davidjames@chromium.org>
Trybot-Ready: David James <davidjames@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: David James <davidjames@chromium.org>
3 files changed