Send a nicer error when patches can't be submitted.
Send a nicer error when patches can't be submitted.

If a CL can't be submitted because a CL it depends on was just rejected
by the CQ, we will just complain that the CL has a dependency "isn't
committed, or marked as Commit-Ready." This error message should be
clarified.

BUG=chromium:310671, chromium:313736
TEST=Unit tests.
TEST=Ran --buildbot --debug run with failing patches.

Change-Id: If8b2af5c9a72a6db38cc113c3b3b86355b5ff50e
Reviewed-on: https://chromium-review.googlesource.com/175306
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite