Remove legacy error downcasting from miner actor

[anorth]

Replaces a swathe of verbose and opaque error conversion with ActorErrors. The general pattern is to always return an ActorError from the state objects, needing minimal context or conversion in the actor code. This is almost all mechanical.

This hasn't purged anyhow entirely from the miner actor, it's still used in some utility modules.

There'll be more of these (smaller) to come. @Stebalien if you review this one, I'll send future iterations on the same pattern only to @alexytsu

See #52.

[anorth]

@Stebalien thanks for reviewing this. I may have been a bit premature. There are some other cases of lost information, but I was trying not to do that.

I will write up an issue with more detail on the challenges.