I would not be inclined to throw an exception here. Generally, I'd prefer the client libraries to be generous in the data they accept. In this particular case, if the resource is a SolidContainer, then the getResources() method must follow the rules of Solid, but if the resource isn't strictly a SolidContainer, it shouldn't cause the consuming application to throw an exception (and thus the getResources method would just return an empty collection).

This condition is specifically to prevent against such cases as this:

A resource is https://example.com/resource and it includes the triple:

<> ldp:contains <https://example.com/resourcechild> .

That triple should not cause the getResources method to include the sibling resource from being included in that list (since it doesn't conform to the Solid definition of containment)