Handle failed discovery rounds and deduplicate findings within a round #63

In the loop-until-dry recipe, finder failures are treated as empty successful rounds, so repeated failures can satisfy the stop condition. Duplicate findings from separate finders in the same round can also be validated more than once. Please return an explicit incomplete-discovery error when any finder fails and add each newly seen key while filtering the round. Reproduce with all finder calls failing, and with two finders returning the same finding key in one round. Related fix: #62.