[roslaunch] Indicating <node> tag when it is actually a <test> tag is…

… confusing.

For example, [this warning msg](https://travis-ci.org/wg-perception/people/jobs/202019288#L3737) (in this [PR](wg-perception/people#49)):

 ```
 * WARN: unrecognized 'group' tag in <node> tag. Node xml is <test name="$(arg testnode_name)" pkg="rostest" test-name="hztest_test" type="hztest">
    <group unless="$(arg expected_success)">
      <node args="-r 0.5 $(find face_detector)/test/face_detector_noface_test_diamondback.bag" name="play" pkg="rosbag" type="play"/>
      <param name="hz" value="0.0"/>
    </group>
```

This is confusing because the tag in question is actually `<test>`, not `<node>`. Although the warning message refers to the exact <test> tag, I didn't try to read it because I was told the issue is with <node>, not <test>.

With this PR the message becomes a bit verbose but hope the readers get better idea.

tools/roslaunch/src/roslaunch/xmlloader.py

@@ -409,7 +409,7 @@ def _node_tag(self, tag, context, ros_config, default_machine, is_test=False, ve
                 elif tag_name == 'env':
                     self._env_tag(t, env_context, ros_config)
                 else:
-                    ros_config.add_config_error("WARN: unrecognized '%s' tag in <node> tag. Node xml is %s"%(t.tagName, tag.toxml()))
+                    ros_config.add_config_error("WARN: unrecognized '%s' child tag in the parent tag element: %s"%(t.tagName, tag.toxml()))
 
             # #1036 evaluate all ~params in context
             # TODO: can we get rid of force_local (above), remove this for loop, and just rely on param_tag logic instead?