Improve external reference resolution

[ewoutkramer]

We can currently have two choices when resolving an external reference to a resource:

• Disable external reference resolution by not providing a resolver. In this case you won't get any errors or warnings.
• Provide a resolver. In this case you either succesfully resolve or else an error is produced by the validator.

It is clear to me that implementers may wish to have behaviours where a set of resources do not need to be resolved, or where we only want to resolve a given set. In this case you may only wish to produce a warning, or maybe nothing at all if the resources is outside the set. This is currently not possible.

I suggest we change the delegate so it returns an enum with the desired error behaviour (just like for extensions) + an out parameter that contains the resolved resource (if any). The delegate than has full control over the behaviour.