Flag ignored service invocations

https://trello.com/c/1QAKfYtf

Distinguish between service invocations that fail and service
invocations that don't run because the service doesn't respond to an
event without returning nil, since nil is an unexpected response in the
middleware.

Author: Melanie Gilman

lib/cc/service.rb

@@ -88,7 +88,7 @@ def receive
         end
       end
 
-      nil
+      { ok: false, ignored: true,  message: "No service handler found" }
     end
 
     private

test/service_test.rb

@@ -26,7 +26,9 @@ def test_nothing_has_a_handler
 
     result = service.receive
 
-    assert_nil result
+    assert_equal false, result[:ok]
+    assert_true true, result[:ignored]
+    assert_equal "No service handler found", result[:message]
   end
 
   def test_post_success