Improve source recipe keyword feature:

* Improve error reporting from sourced recipe;
* Extract cucumber feature from recipe one.

features/recipe.feature

@@ -23,16 +23,3 @@ Feature: recipe evaluation
       \AOK
       recipe.rb:3:.+invalid recipe keyword `invalid_keyword'
       """
-
-  Scenario: source keyword, requires a recipe file
-    Given a recipe with:
-      """
-      source 'sourced_recipe'
-      """
-    And a file named "sourced_recipe.rb" with:
-      """
-      puts 'sourced recipe'
-      """
-    When I execute the recipe
-    Then the exit status must be 0
-    And the output must contain "sourced recipe"