update_engine_client: Clarify error message.

BUG=None
TEST=Built and run on tegra2_dev-board

Review URL: http://codereview.chromium.org/3336016

Change-Id: Ic6a95bacf651aa658ce46d8d08333499d1eb36d3
diff --git a/update_engine_client.cc b/update_engine_client.cc
index a55820a..7bbc62f 100644
--- a/update_engine_client.cc
+++ b/update_engine_client.cc
@@ -49,7 +49,8 @@
                                           kUpdateEngineServiceInterface,
                                           &error);
   if (!proxy) {
-    LOG(FATAL) << "Error getting proxy: " << GetGErrorMessage(error);
+    LOG(FATAL) << "Error getting dbus proxy for "
+               << kUpdateEngineServiceName << ": " << GetGErrorMessage(error);
   }
   *out_proxy = proxy;
   return true;