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.

features/task.feature

@@ -10,3 +10,17 @@ Feature: tasks
     When I execute the recipe
     Then the exit status must be 0
     And the output must contain "hello from recipe"
+
+  Scenario: reports errors for invalid action calls in a task
+    Given a recipe with:
+      """
+      task 'some_task' do
+        invalid_action
+      end
+      """
+    When I execute the recipe
+    Then the exit status must be 70
+    And the output must match:
+      """
+      \Arecipe.rb:2:.+invalid task action `invalid_action'
+      """