Review: a blip swallows no escalation or waiting reminder; replace safely (issue 373)

- A comeback before the clearing was said rings when it is urgent now, and
  says the reminder or escalation that was waiting when it cleared.
- Comebacks never said are counted apart, so "again, Nth time" counts only
  what the operator was told; an urgent comeback after a said clearing rings
  again in place of its message.
- telegram sends the new message before deleting the earlier one, so a
  failed send leaves the operator a message.
- desk-channel keeps a machine's notification on record when its replace
  fails, and says a message shown nowhere as failed.
- A store that cannot be read sends a new message instead of holding it.
- Tests for each, and for a reminder in a burst and a clearing the channel
  could not take.

modules/telegram/README.md

@@ -9,8 +9,9 @@ as the operator.
   buttons — each button carries only the router's one-time ticket — or an edit of a message shown before.
   It sends to the account the router names as linked, or, before one is, to the chat a `/start` came from.
   A message sent under a handle already shown (a reminder, an escalation) **replaces** it (issue 373): the
-  earlier message is deleted and the new one sent, ringing as its kind rings, since an edit here notifies
-  nobody; one the service will not delete loses its buttons and says a newer message follows.
+  new message is sent, ringing as its kind rings (an edit here notifies nobody), and then the earlier one is
+  deleted; one the service will not delete loses its buttons and says a newer message follows. A send that
+  fails leaves the earlier message.
 - **Says what the operator chose** on `intake` under its own kind: a tapped button (`choice`), with the
   sender as Telegram authenticated it; `/start` (`link`); a message it could not deliver (`failed`); and
   whether it can send (`standing`). Only a private chat with the account itself counts.