User rename should return 400 (Bad Request) when renaming to a user that already exists (returns 500 now)

user1 and user2 both exist. If I run:

curl -vX POST 'http://localhost:8095/crowd/rest/usermanagement/1/user/rename?username=user1' -umyapp:myapp -H 'Content-Type: application/json' -d '{ "new-name": "user2" }' 

I get a 500, with the response:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?><error><reason>OPERATION_FAILED</reason><message>User user2 already exists.</message></error>