Do not crash if all resolvers are unreachable

urlchecker_bot.py

@@ -32,7 +32,7 @@ class WebBot:
             self._db, status_id, self.config["DNS"].split()
         )
         if not resolver_dict:
-            raise NotImplementedError("No resolver available")
+            return
 
         # Calculate the full list of domain to check
         domain_list = self.config["DOMAIN"].split()