JS: Provides meaningful message in case of compile issues

modules/js/src/make_umd.py

@@ -103,4 +103,7 @@ if __name__ == "__main__":
     if len(sys.argv) > 2:
         opencvjs = sys.argv[1]
         cvjs = sys.argv[2]
+        if not os.path.isfile(opencvjs):
+            print('opencv.js file not found! Have you compiled the opencv_js module?')
+            exit()
         make_umd(opencvjs, cvjs);