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/client_library/client_dbus.cc b/client_library/client_dbus.cc
index 270a987..0d6b783 100644
--- a/client_library/client_dbus.cc
+++ b/client_library/client_dbus.cc
@@ -225,5 +225,10 @@
                             nullptr);
 }
 
+bool DBusUpdateEngineClient::GetLastAttemptError(
+    int32_t* last_attempt_error) const {
+  return proxy_->GetLastAttemptError(last_attempt_error, nullptr);
+}
+
 }  // namespace internal
 }  // namespace update_engine