DEV: Show parameters on a service contract failure

Now, when calling `StepsInspector#error` on a contract failure, the
output will contain the parameters provided to the contract.

lib/steps_inspector.rb

@@ -73,7 +73,7 @@ class StepsInspector
   # @!visibility private
   class Contract < Step
     def error
-      step_result.errors.inspect
+      "#{step_result.errors.inspect}\n\nProvided parameters: #{step_result.parameters.pretty_inspect}"
     end
   end