["BUGFIX" beta] Improve unhandled action error messages

1) The comment about `return true` being a possible source of error
   is incorrect; even if an action handler returns true, the action
   is considered handled and it won't cause an error even if parent
   routes don't handle the event.
2) 99% of the time, the only time people see the error about no
   active handlers is when they try to call `Route#send` within
   `model`, `beforeModel`, or `afterModel`. Most likely the solution
   is to call `Transition#send` on the transition object passed into
   the various model hooks, so I've updated the error message
   to suggest using `Transition#send` instead.