Diagnosing expected selector in app-start

Test: manual
Bug: 179392551
Change-Id: I4c7173e784caaa6435927f94eef6b269884d0ce9
diff --git a/tests/tapl/com/android/launcher3/tapl/Launchable.java b/tests/tapl/com/android/launcher3/tapl/Launchable.java
index 3fc83ff..c4a566b 100644
--- a/tests/tapl/com/android/launcher3/tapl/Launchable.java
+++ b/tests/tapl/com/android/launcher3/tapl/Launchable.java
@@ -67,7 +67,7 @@
                 () -> "Launching an app didn't open a new window: " + label);
 
         mLauncher.assertTrue(
-                "App didn't start: " + label,
+                "App didn't start: " + label + " (" + selector + ")",
                 TestHelpers.wait(Until.hasObject(selector), LauncherInstrumentation.WAIT_TIME_MS));
         return new Background(mLauncher);
     }