A kludge would be for the completer to return a special completion object in the case of error (rather
than throwing an interactive_error).  The object could have a count of zero (or null) and a "error"
member containing an error message.  It could also specify a get_match_required member as usual.  This
would allow the match_required processing to occur, so that an error will be signalled on pressing enter.
 The error member can be used to display an error from update_completions_done().