Remove fatal in double-claim

[Mark Smith]

This is per @bonafidehan, who ran into the fatal double claim.

We should make it such that the interface for the Consumer channel is that if the channel has closed, the Consumer object (or the Marshal object) has declared itself unhealthy and should be thrown away. The world is in a bad place.

Then we should remove the Fatal and make it a channel close that propagates to all of the Consumers on the same Marshal. Since we can't know who's at fault for the bad state, we need to terminate everything.