Return 'status' field for STATS on deleted LB.

Fixes bug 1177642.

Due to a race condition in some of our Jenkins tests, it is possible
that we could send a STATS message to a LB that has just been deleted.
To recognize this situation, we'll return a FAIL message, but include
a new 'status' field in the JSON response indicating the LB is deleted.

Change-Id: I785cfdff526e67f4b55bf3f9bff911052c27ece7