Failed responses should put information in the response payload body instead of HTTP header

When using Jarvis in REST mode, `req.Fail` can put error information from `⎕EM` or `⎕SI` (depending on `ErrorInfoLevel`) into the HTTP response header as the "reason phrase". However, HTTP clients can ignore the reason phrase, proxies can change it and it was removed in the HTTP/2 spec.

I think the error information should be put as the response body/payload when `req.Fail` is invoked.

Start at the REST-mode implementation of req.Fail and trace how ErrorInfoLevel selects information from ⎕EM or ⎕SI for the HTTP response. Check how failed responses currently set the reason phrase and identify the response payload format used by Jarvis. Done means clients receive the error information in the response body rather than only in the HTTP header.

Written by the indexing model from the issue text.