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

nova/exception.py

@@ -584,6 +584,10 @@ class NoFloatingIpsDefined(NotFound):
     message = _("Zero floating ips exist.")
 
 
+class NoFloatingIpInterface(NotFound):
+    message = _("Interface %(interface)s not found.")
+
+
 class KeypairNotFound(NotFound):
     message = _("Keypair %(name)s not found for user %(user_id)s")