change unresponsive_engines to a set to eliminate duplication of errors

searx/results.py

         self._number_of_results = []
         self._ordered = False
         self.paging = False
-        self.unresponsive_engines = []
+        self.unresponsive_engines = set()
 
     def extend(self, engine_name, results):
         for result in list(results):
             return 0
         return resultnum_sum / len(self._number_of_results)
 
-    def add_unresponsive_engine(self, engine_name):
-        self.unresponsive_engines.append(engine_name)
+    def add_unresponsive_engine(self, engine_error):
+        self.unresponsive_engines.add(engine_error)

tests/unit/test_webapp.py

                                                 corrections=set(),
                                                 suggestions=set(),
                                                 infoboxes=[],
-                                                unresponsive_engines=[],
+                                                unresponsive_engines=set(),
                                                 results=self.test_results,
                                                 results_number=lambda: 3,
                                                 results_length=lambda: len(self.test_results))