Add support for ExceptionMappers

[reactor]

Add support for ExceptionMappers

[jbrisbin]

Exceptions are generally reported into a Reactor by the exception type. There is a Selector class that has a helper function Fn.T that is analogous to the SpEL function of the same name. If you register a Consumer using that Selector, it will do an isAssingableFrom check.

So if you assign an error handler like:

reactor.on(Fn.T(IllegalArgumentException.class), new Consumer<Event<IllegalArgumentException>>() { ... });

That will catch errors that are reported like:

reactor.on(e.getClass(), Fn.event(e));

Which happens in a lot of places in the code.

You can also catch exceptions hierarchically by registering a class higher in the type hierarchy (like Throwable etc...).

[smaldini]

I think we could relate it to #23

[smaldini]

Might add traceErrors() to ReactorSpec and then reroute to Environment.rootReactor()

[ifesdjeen]

If I understand correctly, this logic is already implemented.