rgw: Don't send error body when it's a HEAD request

The main 'operation' know not to send any body for HEAD requests.

However for errors, this was not the case, the formatter would be
flushed and would send the error 'message' in the body in all cases.

For the FastCGI case it doesn't seem to be an issue, it's possible
that the webserver (apache/lighttpd/...) cleans up the response
into shape.

But when using the new civetweb frontend this cause invalid HTTP.

Fixes #8539

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
Signed-off-by: Sylvain Munaut <s.munaut@whatever-company.com>