Display more helpful error when 127.0.0.1 returns 50x

[dstillman]

Since Standalone never returns 50x errors (as far as I know), we should detect them and suggest checking proxy, firewall, or VPN settings to exclude 127.0.0.1.

[adomasven]

We do return 5xx responses these days and all errors point people to translator troubleshooting instructions. Also when blocked by firewalls etc requests to standalone either return 404 or just time out.

Not closing since technically I think we have room for improvement here.