Fixed the duplication of the message. But I don't know how.

@aronasorman this is a potential fix for #1507


I worked out how the message was being generated, and just by experimenting, figured out that the `communicate_api_failure` function was the cause of it. On the knowledgemap it was being generated by the `kmap_editor` making an AJAX request (`get_exercise_logs`) and on the exercise itself it was generated by `exercises.js`. Now that I'm writing this I'm realizing the original message is likely generated by some Django code somewhere, but I searched the codebase for the error message itself and only found them being pieced together in the API. So I'm at a loss as to where the original is being created.

But the fact of the matter is, getting rid of the communicate API failure makes it stop, and I don't think it will hurt anything else. What say you?