Improve source recipe keyword feature:

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

features/source.feature

@@ -0,0 +1,33 @@
+Feature: source recipe keyword
+
+  Scenario: 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"
+
+  Scenario: reports errors from sourced recipes
+    Given a recipe with:
+      """
+      source 'sourced_recipe'
+      """
+    And a file named "sourced_recipe.rb" with:
+      """
+      puts 'OK'
+
+      invalid_keyword
+      """
+    When I execute the recipe
+    Then the output must match:
+      """
+      \AOK
+      sourced_recipe.rb:3:.+invalid recipe keyword `invalid_keyword'
+      recipe.rb:1:.+
+      """