New error handling in poc and features

spec/test/features/data/handler/error_handleridnotfound.feature

@@ -25,4 +25,10 @@ Feature: Fail to retrieve resources from nonexistent handler in Kapow! server.
     Given I have a running Kapow! server
     When I get the resource "/request/path" for the handler with id "XXXXXXXXXX"
     Then I get 404 as response code
-      And I get "Handler ID Not Found" as response reason phrase
+      And the response header "Content-Type" contains "application/json"
+      And I get the following response body:
+      """
+      {
+        "reason": "Handler ID Not Found"
+      }
+      """

spec/test/features/data/handler/error_invalidresource.feature

@@ -27,4 +27,10 @@ Feature: Fail to retrieve an invalid resource for a handler in Kapow! server.
     When I send a request to the testing route "/foo"
       And I get the resource "/invented/path"
     Then I get 400 as response code
-      And I get "Invalid Resource Path" as response reason phrase
+      And the response header "Content-Type" contains "application/json"
+      And I get the following response body:
+      """
+      {
+        "reason": "Invalid Resource Path"
+      }
+      """

spec/test/features/data/handler/error_itemnotfound.feature

@@ -28,4 +28,10 @@ Feature: Fail to retrieve nonexistent resource items in Kapow! server.
     When I send a request to the testing route "/foo"
       And I get the resource "/request/params/meloinvento"
     Then I get 404 as response code
-      And I get "Resource Item Not Found" as response reason phrase
+      And the response header "Content-Type" contains "application/json"
+      And I get the following response body:
+      """
+      {
+        "reason": "Resource Item Not Found"
+      }
+      """