Minor Improvement: Kryo.readObject should detect an abstract class and throw an IllegalArgumentException

[buko]

This is a very minor improvement to provide more helpful error messages:

Describe the solution you'd like

Calls to Kryo.readObject(Input, Class) will throw an exception if passed an abstract class:

com.esotericsoftware.kryo.kryo5.KryoException: Error constructing instance of class: 

This of course makes sense but the error message isn't very helpful. It doesn't actually explain why the the class couldn't be constructed. The underlying java.lang.InstantiationException provides zero information.

A more helpful protocol might be for kryo.readObject would immediately detect that it's being asked to instantiate an abstract class and raise an IllegalArgumentException.

[NathanSweet]

Good candidate for a PR.