feat(debug/ux): display traceback of stderr when exception in addition of the html page

This will make debugging easier and make test debugging /way/ easier because you switch from having, not all the time, only the html containing the traceback with the exception (which doesn't make it very readable) to having /all the time/ the exception in CLI which comes with a lot of advantages:

• you can now read your logs and having debugging information
• you can have the real error when running all the tests that do request to the server (otherwise you don't)
• this is much more readable than the traceback in html form... when you have it