Issue 97 proposal

This is a work in progress... expect a few more commits. Overall this pull request cleans up error handling in the main script runner, the `base.BaseActor` and `group.Sync`/`group.Async` actors. Specifically, the following improvements have been made:

* An Actor failure in a `group.Sync` group will prevent the rest of the actors from executing. Previously, the error would be logged but the rest of the actors would continue to execute.
* Fix how JSON decode failures are handled. Previously a JSON-decode failure would raise an exception, but was not caught properly and generated a stack trace.
* Catch all exceptions to prevent Kingpin from crashing during an Actor execution, but throw a massive warning when its an unexpected exception.
That means that a None would also pass, I think.