actors: Move error propagation within <-wait; add <-wait*, <-reply-wait*.

* 8sync/actors.scm (send-message): Rename xtra-params argument to
send-options and use it in aborting to the hive prompt.
(<-wait*, <-reply-wait*): New procedures.  These allow for adding
a send-options list as a first argument, including #:accept-errors.
These also wrap call wait-maybe-handle-errors.
(<-wait, <-reply-wait): Updated as thin wrappers around <-wait* and
<-reply-wait*.
(wait-maybe-handle-errors): New procedure.
(<waiting-on-reply>): New record type.
(hive-process-message): Update to accept send-options and make use
of <waiting-on-reply>.  Removes error handling, which now hapens in
wait-maybe-handle-errors.