Make sure primeBackend snapshot listener doesn't fail. (#85)

Author: mikelehen

firebase-firestore/src/androidTest/java/com/google/firebase/firestore/testutil/IntegrationTestUtil.java

@@ -124,6 +124,7 @@ private static void primeBackend() {
       ListenerRegistration listenerRegistration =
           docRef.addSnapshotListener(
               (snapshot, error) -> {
+                assertNull(error);
                 if ("done".equals(snapshot.get("value"))) {
                   watchUpdateReceived.setResult(null);
                 } else {