Handle error in associate floating IP (bug 845507)
Handle error in associate floating IP (bug 845507)

* Raise NoFloatingIpInterface when associate floating IP has an
  interface that doesn't exist.
* Return 500 Internal server Error, indicate only that associate
  action failed

Change-Id: I6b032f7554795de64bce8e669d20269210c62e33

n0ano/ganttclient