update_engine: Use helper Command::AbortWithCustomError() method

weaved now provides new variants of Command::Abort() that allow
to specify the error information as brillo::Error or binder::Status.

Command::AbortWithCustomError(brillo::Error*) also makes sure the error
code is prepended with "_", so no need to add that in OnTrackChannel().

BUG: 25875613
Change-Id: Ic9b2cb6f6ef092b85883ebb5bf5e4c936aa770f0
diff --git a/weave_service.cc b/weave_service.cc
index 414234c..c6a67f2 100644
--- a/weave_service.cc
+++ b/weave_service.cc
@@ -101,7 +101,7 @@
 void WeaveService::OnCheckForUpdates(std::unique_ptr<weaved::Command> command) {
   brillo::ErrorPtr error;
   if (!delegate_->OnCheckForUpdates(&error)) {
-    command->Abort(error->GetCode(), error->GetMessage(), nullptr);
+    command->AbortWithCustomError(error.get(), nullptr);
     return;
   }
   command->Complete({}, nullptr);
@@ -111,7 +111,7 @@
   string channel = command->GetParameter<string>("channel");
   brillo::ErrorPtr error;
   if (!delegate_->OnTrackChannel(channel, &error)) {
-    command->Abort(error->GetCode(), error->GetMessage(), nullptr);
+    command->AbortWithCustomError(error.get(), nullptr);
     return;
   }
   command->Complete({}, nullptr);