FIX text/html type for Debug::friendlyError() in live mode

Avoid unparsed HTML responses in case of httpError(4xx) or httpError(5xx) in
live mode. The SS_HTTPResponse_Exception constructor automatically set
"text/plain" a content type, which conflicts with the later choice of
Debug::friendlyError() to return HTML.

This is a hotfix, needs more work to properly separate logic flow from
presentation concerns (e.g. through request processors).