Errorhandling when referencing non-entities in collections.

[ilikeorangutans]

Right now when non-entity, i.e. a class that lacks the @entity annotation, is referenced in a collection, the MappingExtractor will crash with a confusing exception:

org.om.core.api.exception.MappingException: Target type interface java.util.List is not an entity. (in com.foo.type.that.ReferencesTheNonEntity)
    at org.om.core.impl.mapping.extractor.FieldMappingExtractorImpl.extract(FieldMappingExtractorImpl.java:88)
    at org.om.core.impl.mapping.extractor.EntityMappingExtractorImpl.extractFields(EntityMappingExtractorImpl.java:66)
    at org.om.core.impl.mapping.extractor.EntityMappingExtractorImpl.extract(EntityMappingExtractorImpl.java:43)
    at org.om.core.impl.mapping.registry.OnDemandMappingRegistry.getMapping(OnDemandMappingRegistry.java:25)
    at org.om.core.impl.session.ImmutableSessionImpl.get(ImmutableSessionImpl.java:56)

To reproduce:

Type A has a collection mapping

@Entity
class A {
  @Collection(targetType=B.class)
  private List<B> bs;
}

However, B is not an entity:

class B {   }

The error message should be more clear on what happens and provide guidance how to resolve the situation.