Rally not show rightful error message at config validation

Rally show poor error message instead of rightful error message when ambiguous
config of image name.

reproduce:
sample glance image list:
+--------------------------------------+---------+
| ID | Name |
+--------------------------------------+---------+
| 0a8a021b-f2f7-427e-90e4-06d2a99f3b45 | cirros4 |
| 6d588e19-9e5a-4ff9-bd56-ef3615db7330 | cirros3 |
+--------------------------------------+---------+

sample scenario.json:
{
    "NovaServers.boot_and_delete_server": [
        {
            "args": {
                "flavor": {
                    "name": "m1.nano"
                },
                "image": {
                    "name": "^cirros.*"
                },
                "force_delete": false
            },
            "runner": {
                "type": "constant",
                "times": 10,
                "concurrency": 2
            },
            "context": {
                "users": {
                    "tenants": 3,
                    "users_per_tenant": 2
                }
            }
        }
    ]
}

Run "rally task start scenario.json" then got the following error message.

Invalid scenario argument: 'Image '{'name': '^cirros.*'}' not found'

But, it should be the following error message.

Invalid scenario argument: '{'message': "Image with name '^cirros.*' is
ambiguous, possible matches by id: 0a8a021b-f2f7-427e-90e4-06d2a99f3b45,
6d588e19-9e5a-4ff9-bd56-ef3615db7330"}'

The same applied to flavor.