Fix type of unresponsive_engines

Previously __get_translated_errors
returned a list. But unresponsive_engines
is a set.

Closes #2305

searx/webapp.py

@@ -735,12 +735,12 @@ def search():
 
 
 def __get_translated_errors(unresponsive_engines):
-    translated_errors = []
+    translated_errors = set()
     for unresponsive_engine in unresponsive_engines:
         error_msg = gettext(unresponsive_engine[1])
         if unresponsive_engine[2]:
             error_msg = "{} {}".format(error_msg, unresponsive_engine[2])
-        translated_errors.append((unresponsive_engine[0], error_msg))
+        translated_errors.add((unresponsive_engine[0], error_msg))
     return translated_errors