Improve error reporting during recipe evaluation

* 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.

lib/producer/core.rb

@@ -1,5 +1,6 @@
 require 'producer/core/cli'
 require 'producer/core/env'
+require 'producer/core/errors'
 require 'producer/core/recipe'
 require 'producer/core/recipe/dsl'
 require 'producer/core/task'