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).

This doesn't affect 3.0 since the commit causing the regression was 3.1 only: 39952f4a5ca313491984f9ef7d4d00ee4a41b23a