A special error variant for improbable cases

[fjarri]

For some protocols, it is possible to have a situation in finalize() which is a fault, not attributable (because it's not a result of someone's malicious action), but whose probability is statistically negligible. For example, if in a KeyGen the secret shares generated by the nodes add up to zero.

What should we do for such situations? Just return LocalError? Have a special error variant which will mean for the user "just restart the protocol" as opposed to them getting a LocalError and halting the node because they think it's a bug?