Make errors from ArticleSave hooks propagate to the interface

Make errors from ArticleSave hooks propagate to the interface

Aborting a save from the ArticleSave hook and putting an error in
$status didn't actually propagate the error message to the user, but
instead displayed the edit conflict page (!). Fix this so that if we get
an unrecognized error from ArticleSave, we treat it as an extension
error and render it rather than going into conflict mode.

Similarly, make the API attempt to render the error through
dieUsageMsg() like it already does for AS_END

Change-Id: Iccf78480240d0c7ed321438c8190472805957099