Refactor producer invocation in cucumber features:

Use "When I successfully execute" instead of "When I execute", and
assert that exit status code was successful in this new step.

features/recipes/evaluation.feature

@@ -5,6 +5,5 @@ Feature: recipe evaluation
       """
       puts 'hello from recipe'
       """
-    When I execute the recipe
-    Then the exit status must be 0
+    When I successfully execute the recipe
     And the output must contain exactly "hello from recipe\n"

features/recipes/source.feature

@@ -11,6 +11,5 @@ Feature: `source' recipe keyword
       """
       puts 'sourced recipe'
       """
-    When I execute the recipe
-    Then the exit status must be 0
+    When I successfully execute the recipe
     And the output must contain "sourced recipe"

features/recipes/target.feature

@@ -7,6 +7,5 @@ Feature: `target' recipe keyword
 
       puts env.target
       """
-    When I execute the recipe
-    Then the exit status must be 0
+    When I successfully execute the recipe
     And the output must contain exactly "some_host.example\n"