Add a feature to get the last UpdateAttempt ErrorCode from update_engine
For autotest, update_engine test failures are always hard to debug,
since the error message is not clear. Add a new flag 'last_attempt_error'
to get the last UpdateAttempt ErrorCode from update_engine.
Bug:25598547
Test:emerge-peppy update_engine
emerge-peppy update_engine_client
cros flash a test image to DUT.
(on the DUT):update_engine_client --last_attempt_error
Compare the results with the update_engine logs, matched.
Change-Id: Id12681097ed30b0826cad68809f17f934a07e5b2
20 files changed