Add resultMessage for class constraint

[thomasdegroot18]

Thanks for continuing to improve this library!

Observation

Currently the class constraint component produces no message at all. This is a missed opportunity. Together with issue 63 Adding a result message would tie up all of the default constraints.

Expectation

I would propose tweaking the detail validation result message when an Class constraint is violated.

A proposed message could be of type, :

Value is not of type {$class}

Where $class cuold be the rdfs:label if there is a label and otherwise the node value if there is no label is not given.