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/after-ecj/NonNullWithAlternateException.java

@@ -6,14 +6,14 @@ public class NonNullWithAlternateException {
   public void testMethod(@lombok.NonNull String arg) {
     if ((arg == null))
         {
-          throw new java.lang.IllegalArgumentException("arg");
+          throw new java.lang.IllegalArgumentException("arg is null");
         }
     System.out.println(arg);
   }
   public @java.lang.SuppressWarnings("all") void setTest(final @lombok.NonNull String test) {
     if ((test == null))
         {
-          throw new java.lang.IllegalArgumentException("test");
+          throw new java.lang.IllegalArgumentException("test is null");
         }
     this.test = test;
   }