agenda: Make agendas run with "easier" default arguments

* 8sync/agenda.scm (make-agenda): Set #:pre-unwind-handler to
  print-error-and-continue.  This makes it so that by default,
  exceptions get printed to the shell in which the agenda was spawned.
  (start-agenda): Set #:stop-condition to stop-on-nothing-to-do.  This
  is a bit heavier than maybe desirable but is a good "default" stop
  condition, and prevents the agenda from entering into a busy-loop
  when computation has finished.