make _on_parameter_event return result correctly (#817)

* make _on_parameter_event return result correctly

Previously, `_on_parameter_event` always returned `successful=True` to the
caller (e.g., ros2param set) regardless of whether setting `use_sim_time`
parameter actually succeeded or not.

* PoC:
```sh
# terminal 1
$ ros2 run examples_rclpy_minimal_publisher publisher_member_function

[INFO] [1629490410.452032755] [minimal_publisher]: Publishing: "Hello World: 0"
[INFO] [1629490410.918999697] [minimal_publisher]: Publishing: "Hello World: 1"
[INFO] [1629490411.419087028] [minimal_publisher]: Publishing: "Hello World: 2"
[INFO] [1629490411.919343319] [minimal_publisher]: Publishing: "Hello World: 3"
[INFO] [1629490412.419345165] [minimal_publisher]: Publishing: "Hello World: 4"
[INFO] [1629490412.919260702] [minimal_publisher]: Publishing: "Hello World: 5"
[ERROR] [1629490413.030775970] [minimal_publisher]: use_sim_time parameter set to something besides a bool
[INFO] [1629490413.419389164] [minimal_publisher]: Publishing: "Hello World: 6"
[INFO] [1629490413.919106545] [minimal_publisher]: Publishing: "Hello World: 7"
```

```sh
# terminal 2
$ ros2 param set /minimal_publisher use_sim_time Trueeeee

Set parameter successful
```

As demonstrated above, when trying to set `use_sim_time` param to an invalid
type, the minimal_publisher node complains it cannot. However, ros2 param
prints "Set parameter successful". This commit fixes this issue.

Signed-off-by: Seulbae Kim <squizz617@gmail.com>

* use single quote

Signed-off-by: Seulbae Kim <squizz617@gmail.com>

Co-authored-by: Shane Loretz <sloretz@osrfoundation.org>

rclpy/rclpy/time_source.py

@@ -123,19 +123,24 @@ def clock_callback(self, msg):
             clock.set_ros_time_override(time_from_msg)
 
     def _on_parameter_event(self, parameter_list):
+        successful = True
+        reason = ''
+
         for parameter in parameter_list:
             if parameter.name == USE_SIM_TIME_NAME:
                 if parameter.type_ == Parameter.Type.BOOL:
                     self.ros_time_is_active = parameter.value
                 else:
+                    successful = False
+                    reason = '{} parameter set to something besides a bool'.format(
+                        USE_SIM_TIME_NAME)
+
                     node = self._get_node()
                     if node:
-                        node.get_logger().error(
-                            '{} parameter set to something besides a bool'
-                            .format(USE_SIM_TIME_NAME))
+                        node.get_logger().error(reason)
                 break
 
-        return SetParametersResult(successful=True)
+        return SetParametersResult(successful=successful, reason=reason)
 
     def _get_node(self):
         if self._node_weak_ref is not None: