* Throw symbols instead of raising exceptions to test code evaluation,
this will reduce "bad" usages of exceptions, symbols seems a little
better for now;
* Add a specific exception to be used in specs for testing behaviour
when an exception is raised from a recipe.
* Report invalid action usages from tasks;
* Implement backtrace cleaning in CLI;
* Extract error class declarations in a new errors file;
* Replace raise with fail keyword in task DSL class.
