ResourceHandler should have the option to call a "next" handler instead of 404 on missing resource

A ResourceHandler always must be the end of a handler chain because if it fails to find a resource it send an error on the exchange and closes it. It should allow another handler as fallback instead.

This means that the ResourceManager might need a new method isExists() which can be called before switching to blocking mode. Therefore can chain to a fallback handler. Although I'm not sure all ResourceManagers would be blocking either, but in this case the chain should be able to happen before blocking is forced.

A use case, we receive a list of separate directories to unify into one virtual directory, they are overlaid, so one might contain site ./scripts and ./css another might contain html files, and another might contain the contents of webjars ... and they all need to act as-if they were merged together. It is not possible now due to the forced 404 when the first resource manager fails.