Indicate registration failed due to email address being in use

[aggied]

I am trying out this repo on a small side project, and I might get around to a PR, but I wanted to to flag this for anyone thinking of using this repo and/or working on their own PR request.

In my testing I could not register the same email twice (good), but there was no indication to the user that this was the case. It wasn't in CloudWatch either but since that doesn't seem like an application error to me I might opt to leave it out as well.