Elmattic/actors review f20

[elmattic]

Summary of changes
Changes introduced in this pull request:

• Move validation code out of the for_each loop

Reference issue to close (if applicable)

Closes #1286

Other information and links

Note this still doesn't match exactly one for one Go actor logic. E.g. if we return at this point we never touch the state since we update it afterwards. In contrary Go actor updates the state in the for_each loop.

[arajasek]

This seems correct to me. Note this is a somewhat cosmetic fix -- if validate state fails, the chain halts (and so a subtle distinction between how Go and Rust behaved is immaterial).

Also note that this is only called by Cron, so gas is irrelevant (cron is gasless).