Associate an already associated Floating IP to server is returning 500 error code

Association of a floating IP to server where floating IP is already associated to another server is returning 500 error code

Steps to reproduce:

1. Create two instances 'a' and 'b' (wait till they are ACTIVE)
2. Create a floating IP
3. Associate floating IP to instance 'a'
4. Associate the same floating IP to instance 'b'

Expected result:
The error message should state that "This acction is not allowed as floating IP is already associted to 'a'." and should return 400 as it is a BadRequest.

Actual Result:
ERROR: Associate floating ip failed (HTTP 500)

LOG:

[muralik@openstack27 ~/tempest_16may/tempest/tempest/tests]$nova list
+--------------------------------------+--------+--------+--------------------------------+
| ID | Name | Status | Networks |
+--------------------------------------+--------+--------+--------------------------------+
| 9cc6e27b-9cc8-4b9d-b116-5470773cc286 | test-2 | ACTIVE | private=10.0.0.4 |
| ff0b8b93-6377-485b-9c42-59d8e7d1cfec | test-1 | ACTIVE | private=10.0.0.2, 172.24.4.225 |
+--------------------------------------+--------+--------+--------------------------------+
[muralik@openstack27 ~/tempest_16may/tempest/tempest/tests]$nova add-floating-ip 9cc6e27b-9cc8-4b9d-b116-5470773cc286 172.24.4.225

REQ: curl -i http://10.233.52.27:8774/v2/732001bbd21942f1bec893c67c850066/servers/9cc6e27b-9cc8-4b9d-b116-5470773cc286/action -X POST -H "X-Auth-Project-Id: demo" -H "User-Agent: python-novaclient" -H "Content-Type: application/json" -H "Accept: application/json" -H "X-Auth-Token: 68b79689a0234e009df67e281af4c266"

REQ BODY: {"addFloatingIp": {"address": "172.24.4.225"}}

RESP:{'date': 'Thu, 17 May 2012 11:13:13 GMT', 'status': '500', 'content-length': '74', 'content-type': 'application/json; charset=UTF-8', 'x-compute-request-id': 'req-2d9d9f2c-8304-48dc-bc17-626925ac1003'} {"computeFault": {"message": "Associate floating ip failed", "code": 500}}

ERROR: Associate floating ip failed (HTTP 500)