[appmgr] improve error message when unable to find entry point

Have appmgr say the URL of the component it has failed to start when
there is an issue finding the described entry point of the component.

TEST: modified message looks correct when launching a bad component
Change-Id: Ifdd9fca13e61affc93d590f28b24f9bde89151f3
diff --git a/bin/appmgr/realm.cc b/bin/appmgr/realm.cc
index cf6b6bb..2f45637 100644
--- a/bin/appmgr/realm.cc
+++ b/bin/appmgr/realm.cc
@@ -683,7 +683,8 @@
     TRACE_DURATION_END("appmgr",
                        "Realm::CreateComponentFromPackage:VmoFromFilenameAt");
     if (!app_data) {
-      FXL_LOG(ERROR) << "component has neither runner (error: '"
+      FXL_LOG(ERROR) << "component '" << package->resolved_url.get()
+                     << "' has neither runner (error: '"
                      << runtime_parse_error << "') nor elf binary: '"
                      << bin_path << "'";
       component_request.SetReturnValues(kComponentCreationFailed,