Make error message more user-friendly when non-admin user tries to log in

Closes #87 (closed)

mentioned in commit c7ebd700

merged

Actually I think it should maybe link to https://docs.pleroma.social/backend/administration/CLI_tasks/user/#set-the-value-of-the-given-users-settings as the command depends if OTP or Source is used.

Done in !111 (merged)