bark: don't crash on no-mail summary saving

Instead, give a %no-mail response message. It's not entirely unexpected
to encounter ships that don't have a hosting account email address
associated with them, so it shouldn't be a reason to crash.

The crash is particularly problematic because as of the previous
commits, we now actually show the trace of thread crashes.

desk/ted/save-summary.hoon

@@ -35,7 +35,8 @@
 ::
 =/  [%khan %arow %.y %noun vs=vase]  mine
 =+  !<(mail=(unit cord) vs)
-?>  ?=(^ mail)
+?~  mail
+  (pure:m !>('no-mail'))
 ;<  ~  bind:m
   %-  send-raw-card
   :*  %pass  /update-merge-fields/(scot %p ship.args)