fix error handling in +make-join
::
?. ?=([~ %success [%success ^ ^] %success %path *] initial-result)
(wrap-error initial-result)
+ ?. ?=([%success *] head.head.u.initial-result)
+ (wrap-error `head.head.u.initial-result)
+ ?. ?=([%success *] tail.head.u.initial-result)
+ (wrap-error `tail.head.u.initial-result)
::
=/ first-cage=cage (result-to-cage head.head.u.initial-result)
=/ second-cage=cage (result-to-cage tail.head.u.initial-result)
