replace run_after with order; do not run after cbs on exception

[jph00]

fastai callbacks previously used run_after and run_before to organize themselves to run in a reasonable order. However, since callbacks can't know what other callbacks exist in either libraries or modules, this wasn't really flexible enough.

Therefore, an int attribute called order is now used to sort callbacks before calling them. To get similar behavior to run_after and run_before you can instead use Foo.order-1 or Foo.order+1 (e.g to order compared to a callback called Foo).