bwauth code needs to be smarter about failed circuits

In the current bandwidth authority code, when a fetch attempt fails, it will still be counted as a circuit that went through all of the nodes -- even if those nodes weren't responsible for the failure.

This has the potential of resulting in a relay not being measured sufficiently, or at all: the code will consider failures from unstable nodes to be relevant for nodes that are perfectly stable.

In slices where exits and entries aren't well-distributed (like, all of them) this can result in some nodes not being measured at all, and losing their consensus weight. This seems to affect exits a lot more than it does other relay types: people on tor-relays@ have mentioned that removing their exit policies gets their consensus weight back, and I have been able to reproduce this.