Give more detailed errors for class conflicts

[csssuf]

When trying to schedule a lot of classes with only one or two sections each, it'd be super useful to be told which classes are conflicting and blocking schedule generation.

[devinmatte]

This should be fairly doable. I think we currently just check to make sure no times overlap, so we could easily return the first two conflicting classes in the error message when the resulting schedules is being returned as 0.