Response contents contain useful information when debugging Gerrit
transient errors.

For example, trying to land a WIP CL fails with a 409 Conflict status,
which is not very useful when trying to troubleshoot.
The message returned by Gerrit makes it clearer:

  Failed to submit 1 change due to the following problems:
  Change XXXXXXX: Change XXXXXXX is work in progress

Bug: 1136295
Change-Id: I7e0036785cb3146eae49e29d3c4fa3e156ae7c20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2463987Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>