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
diff --git a/payload_state_interface.h b/payload_state_interface.h
index 40a13dd..68798ee 100644
--- a/payload_state_interface.h
+++ b/payload_state_interface.h
@@ -192,6 +192,7 @@
   // Sets/gets the P2P download URL, if one is to be used.
   virtual void SetP2PUrl(const std::string& url) = 0;
   virtual std::string GetP2PUrl() const = 0;
+  virtual ErrorCode GetAttemptErrorCode() const = 0;
 };
 
 }  // namespace chromeos_update_engine