Workers handle errors better, code is cleaner

Concordance can now track when its worker fails. Scorer cannot, but doing it right
(both of them) may involve a sort of ticketing system for each job associated with
a concordance or a run.

This commit also fixes many issues and improves (& corrects) the validation module
and associated code.

Related to #15, but to handle #15 well requires tracking jobs better. Not sure it's 
worth the energy expenditure right now to do that. 
I don't know if this is a problem, or just my python-ignorance, but I really don't understand this code.