Improve EntityInstantiator error message if the to be instantiated type is abstract

[mp911de]

Right now, the failure looks like:

java.lang.InstantiationError: com.example.demo.CustomDto
	at com.example.demo.CustomDto_Instantiator_ddeq8t.newInstance(Unknown Source) ~[main/:na]
	at org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator$EntityInstantiatorAdapter.createInstance(ClassGeneratingEntityInstantiator.java:238) ~[spring-data-commons-2.4.5.jar:2.4.5]
	at org.springframework.data.mapping.model.ClassGeneratingEntityInstantiator.createInstance(ClassGeneratingEntityInstantiator.java:87) ~[spring-data-commons-2.4.5.jar:2.4.5]