add new type of exception that may be thrown during creation of publisher/subscription (#1026)

Signed-off-by: Miaofei <[email protected]>

Author: mm318

rclcpp/include/rclcpp/qos_event.hpp

@@ -93,7 +93,11 @@ class QOSEventHandler : public QOSEventHandlerBase
     event_handle_ = rcl_get_zero_initialized_event();
     rcl_ret_t ret = init_func(&event_handle_, parent_handle, event_type);
     if (ret != RCL_RET_OK) {
-      rclcpp::exceptions::throw_from_rcl_error(ret, "could not create event");
+      if (ret == RCL_RET_UNSUPPORTED) {
+        rclcpp::exceptions::throw_from_rcl_error(ret, "event type is not supported");
+      } else {
+        rclcpp::exceptions::throw_from_rcl_error(ret, "could not create event");
+      }
     }
   }