Changed the lombok.nonNull.exceptionType configkey to be an enum, partly to enforce only sensible exceptions and partly to make it possible to have the message be ‘x is null’ when throwing IAEs.

test/transform/resource/before/NonNullWithAlternateException.java

@@ -1,4 +1,4 @@
-//CONF: lombok.nonNull.exceptionType = java.lang.IllegalArgumentException
+//CONF: lombok.nonNull.exceptionType = IllegalArgumentException
 
 public class NonNullWithAlternateException {
 	@lombok.NonNull @lombok.Setter private String test;