Mock runtime should not return ActorError for expectation violations

[anorth]

The mock runtime returns ActorError with bogus exit codes in the case of some expectation violations. This is totally wrong – it should immediately fail the test instead. The mock should return an ActorError only in those cases where it's replicating an ActorError returned by the FVM runtime layer.

This is probably an artefact of the Runtime interface methods returning ActorError, which for the most part is inappropriate.

cc @dignifiedquire @Stebalien