Forward error from router

Not sure what you had in mind for error handling (see [todo's](https://github.com/Turistforeningen/leaflet-routing/blob/gh-pages/src/L.Routing.js#L291)), but here is my proposal to pass router errors to the client/caller (e.g. to display an error message):

* pass err from router to callback in ``_routeSegment``
* collect first occuring error for multiple calls and pass to callback and events (``routeWaypoint``, ``rerouteAllSegments``)