Detailed "human readable" diagnosis

[popmonkey]

failed schedule diagnosis is not an unreadable mess anymore and includes advice.

e.g.

--- Infeasibility Diagnosis ---
 - Stint 32: No one could drive without being Unavailable (Brandon)
 -    -> Advice: Mark Brandon as Available during this window.
 - Stint 26: No one could drive without being Unavailable (Cesar)
 -    -> Advice: Mark Cesar as Available during this window.
 - Stint 36: No one could drive without being Unavailable (Jay)
 -    -> Advice: Mark Jay as Available during this window.