Use custom error types and remove error downcasting

[Stebalien]

Currently, we have a lot of "anyhow" errors and rely on error downcasting to figure out the right exit code. Unfortunately, this makes it very difficult to figure out what the exit code should be.

Instead, we'd ideally:

1. Abort immediately once we know the right exit code (power actor tests part 3 #282).
2. Return situation-specific errors from state types like partitions & deadlines, so we can figure out the correct exit code.

[jennijuju]

from zen: make sense to do before the audit, but shouldn't be a blocker

[anorth]

#197 , #214 does a heap of the work for this (not yet merged, needs stewardship)