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/update_attempter.cc b/update_attempter.cc
index bc06502..dd7c1be 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -1098,7 +1098,7 @@
     brillo::Error::AddTo(error,
                          FROM_HERE,
                          brillo::errors::dbus::kDomain,
-                         "_set_target_error",
+                         "set_target_error",
                          error_message);
     return false;
   }