msg: fix response code on exhausted endp resources

When all endpoints are seized and a call agent tries to allocate
another one, then 500 is returned as response code, which means
that the endpoint was not found. This does not match.

- Return 403 which is defined as "Insufficient resources available
  at this time"

Change-Id: Idf241b47e711e35e9f9b5a43f3cea5c0298ea30b
1 file changed