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).
Merge pull request #2674 from chillu/pulls/friendlyerror-contenttype

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