Make "already signed up to SAMS with another login method" error more user-friendly

[vdavid]

Make this error message more understandable / actionable:

To reproduce (needs a clean slate with one's accounts):

1. Go to sourcegraph.com/search, not logged in.
2. Log in with a GitHub account.
3. Log out.
4. Log in with a Google or GitLab account that has the same email address as the GitHub account.
5. Get this error.

Problem: The error message is somewhat informative, but not user friendly, and there is no clear action the user can perform, in form of a button.

The reason for this is that this message is rendered by the back end, there is no front end wired up to this outcome. The solution would be to wire up the front end to this, and design a very simple page with easy-to-understand info, maybe even context on what other sign-in method the user used before, and a clear action.