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/errors.rb

@@ -0,0 +1,8 @@
+module Producer
+  module Core
+    Error                 = Class.new(StandardError)
+    ConditionNotMetError  = Class.new(Error)
+    RecipeEvaluationError = Class.new(StandardError)
+    TaskEvaluationError   = Class.new(RecipeEvaluationError)
+  end
+end