Bug 1233706 - Make error message more helpful for instrumented Robocop tests; r=jchen

MozReview-Commit-ID: 286GgFXm0vR

--HG--
extra : rebase_source : 37d8583b41cb51105752e1ac99c42e6449862ba0

mobile/android/tests/browser/robocop/src/org/mozilla/gecko/FennecNativeDriver.java

@@ -449,7 +449,9 @@ public class FennecNativeDriver implements Driver, CompositorController.GetPixel
 
     public static void log(LogLevel level, String message, Throwable t) {
         if (mLogFile == null) {
-            throw new RuntimeException("No log file specified!");
+            throw new RuntimeException(
+                    "No log file specified!\n" +
+                    "(If you are running this test from an IDE don't forget to before run \"robocop --serve\")");
         }
 
         if (level.isEnabled(mLogLevel)) {