dbus-constants: add a few shill error strings

This adds names for finer-grained failures
including distinguishing between success and
unknown failure.

BUG=b:27767401
TEST=none

Change-Id: Ie449fafc469e36f6ed99ca0868ad1d56d035cc28
Reviewed-on: https://chromium-review.googlesource.com/336898
Commit-Ready: Luigi Semenzato <semenzato@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Samuel Tan <samueltan@chromium.org>
1 file changed