authentication failure text gives no next steps

Created by: terinjokes

• Sourcegraph version: 2.8.9 (server)
• Platform: n/a

Steps to reproduce:

1. Connect to the authentication callback with an unexpected value

Expected:

Error message would explain the failure (invalid state), attempt to fix the situation (unset cookies), and give next steps ("try logging in again" or "delete your cookies for $APP_HOST")

Actual:

Authentication failed. OpenID Connect state paramater did not match the expected value (possible request forgery).