Add `--check-status` flag for TReq exit with an error if the HTTP status code indicates one.

[talis-fb]

By default, TReq exits with 0 when no network or other fatal errors occur. This flag instructs TReq to also check the HTTP status code and exit with an error if the status indicates one.

When the server replies with a 4xx (Client Error) or 5xx (Server Error) status code, TReq exits with 4 or 5 respectively.