RAPI: format error messages as JSON

This patch changes the format of the HTTP error messages from text/html, which
is hard to parse from RAPI clients, to JSON which can be automatically parsed.

The error message is an object, which contains always three keys:
  - code, an integer with the error code
  - message, a short description
  - explain, holding (if available) a description of the error

In order to implement this, there is a bit of change to the http server
and executor classes. I've tested and the error handling still works
(but less optimal, no error message) in case the error formatting itself
raises an exception.

Reviewed-by: imsnah