Properly set http status code for "regular" flask errors (like 405 me…

…thod not allowed), instead of setting them all to 500.

locust/web.py

@@ -147,7 +147,7 @@ def __init__(
         def handle_exception(error):
             error_message = str(error)
             logger.log(logging.CRITICAL, error_message)
-            return make_response(error_message, 500)
+            return make_response(error_message, getattr(error, "code", 500))
 
         @app.route("/assets/<path:path>")
         def send_assets(path):