Add prioritization of configured error message over remote result.

There needs to be a way to say: "no matter what the remote call gives you, please use the error message that I provided, instead of the response from the back end." I do this by saying, if the error message configured starts with an exclamation mark (!), use it (after removing the exclamation mark), otherwise, feel free to use the response.

I am sorry I closed the previous pull request, I did not realize I could simply re-push my changes to it :-( I need to learn more about how github works.