Provide a more specific fail-fast message for missing Entity classes

If an Entity class is not properly registered in the SolverConfig, the fail-fast message gives no guidance by providing a general message that the class is not a valid planning entity.

See:
Exception in thread "main" java.lang.IllegalArgumentException: The entityClass (class com.Foo) has an @InverseRelationShadowVariable annotated property (nap) with a sourceClass (class com.Bar) which is not a valid planning entity.

Caused by:
SolverConfig()
.withSolutionClass(Something::class.java)
.withEntityClasses(Foo::class.java)
.withEntityClasses(Bar::class.java) // overrides the previous line

Proposal:
the message could be slightly extended to guide the user better by adding the "Maybe" sentence about registering the class in the SolverConfig.