When OAuth password verification fails, return 401 instead of redirect (#5111)

When OAuth password verification fails, return 401 instead of redirect (#5111)

Call to warden.authenticate! in resource_owner_from_credentials would
make the request redirect to sign-in path, which is a bad response for
apps. Now bad credentials just return nil, which leads to HTTP 401
from Doorkeeper. Also, accounts with enabled 2FA cannot be logged into
this way.