Update exception messages.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5616 2fea6a4d-a838-0410-917b-93a53c48e9d2
Set extra headers, override content-type when needed.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5617 2fea6a4d-a838-0410-917b-93a53c48e9d2