Raise a helpful exception when an operation returns no response

[aldesantis]

Sometimes, an operation stops the execution earlier than expected and doesn't return a response. This can happen because one of the steps returns a falsey value when it shouldn't. Right now this kind of situation is very complicated to debug because you get a cryptic error message (NoMethodError (undefined method headers' for nil:NilClass)`) and no indication of what went wrong.

Instead, when an operation has no result.response, we could raise a helpful exception from pragma-rails and maybe even integrate with pragmarb/pragma#24 to tell the user where the execution stopped.