iRule TCL Error and Fallback Host

From K6510: BIG-IP system fallback host behavior when a host is down:

When a Fallback Host is defined, and the connection attempt to the selected load balancing target has failed by the time the BIG-IP LTM system is ready to send the request, the BIG-IP LTM system sends the specified HTTP redirect in the following situations:

   - A pool member has not been chosen (the system failed to select a pool or a pool member, or no pool members were available)
   - A pool member is unreachable (when no route to the target exists)
   - A pool member is unresponsive (resets a connection request)
   - A pool member closes an established connection before the request is sent

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