Add message to exception.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1546206 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/collections4/iterators/PeekingIterator.java

@@ -151,7 +151,7 @@ public class PeekingIterator<E> implements Iterator<E> {
      */
     public void remove() {
         if (slotFilled) {
-            throw new IllegalStateException();
+            throw new IllegalStateException("peek() or element() called before remove()");
         }
         iterator.remove();
     }