Only allow 2 exceptions to be automatically raised inside Tasks

The change minimizes the impact of #243. 

Currently all `Celluloid::Error` exceptions are automatically raise inside `Task`s. 
This can potentially cause issues where an Error is incorrectly passed back out of the `Task` as a raised exception, rather than a value. 

There is a potential confusion here between `Celluloid::TimeoutError` and `Celluloid::Task::TimeoutError`. 

*Note*: The timeout PR (#243) reused the same exception class for two different behaviors.