Tell user their email is associated with GitHub account earlier in sign up workflow

If a user has an email, in my case `oscarbatoritesting@gmail.com` that is associated with a DoltHub account via GitHub auth, and they try and use this email to create an account the following happens:
- they are prompted to choose a username
- upon choosing a username they are told the email is associated with another account

We should intervene before we prompt them to choose a username, and tell them the email is associated with an account via GitHub. I encountered this by accident because I forgot that, but was signed into my test Gmail account and try to use it to auth, forgetting I had to do so via GitHub.

Start by reproducing the sign-up workflow with an email already associated with a DoltHub account through GitHub authentication. Trace the flow before the username prompt and identify where the existing-account check occurs. Done means the user is told before choosing a username that the email must be accessed through GitHub authentication.

Written by the indexing model from the issue text.