Private mode requires authentication" error message is not discoverable

Created by: attfarhan

• Sourcegraph version: "Sourcegraph.com
• Platform information: MacOS, Chrome

Steps to reproduce:

1. Point your browser extension to sgdev.
2. Sign out of sgdev. Sign in with OpenIDConnect.
3. On GitHub or GitLab, go to a file in a repository not on sgdev (e.g. if you're pointing your repo to sgdev, go to https://github.com/tsenart/vegeta/blob/master/internal/cmd/echosrv/main.go)
4. Hover over a token, and see that a line appears above the token. Open the browser console and see 401 errors that say "private mode requires authentication". I was not sure of what to do, but remembered we had a GitHub sign-in option, which I used, and solved the problem.

Expected behavior:

The line above the token when hovering should not appear, or a hover should appear showing the error message "private mode requires GitHub/GitLab/CodeHost authentication" or something more specific.

Actual behavior:

The line above tokens appear when hovering and the ambiguous "private mode requires authentication" message appears in the console.