Change exception type to RuntimeError since check happens at runtime

src/parameter_sweep/parameter_sweep.py

@@ -314,7 +314,7 @@ def _publish_updates(self, iteration, solve_status, solve_time):
             return
 
         if not requests_available:
-            raise ImportError(
+            raise RuntimeError(
                 "requests (parameter_sweep optional dependency) not installed"
             )