add exception message

Author: yuzawa-san

iabgpp-encoder/src/main/java/com/iab/gpp/encoder/datatype/encoder/IntegerBitSet.java

@@ -48,7 +48,7 @@ public int size() {
   private int getOffset(int value) {
     int offset  = from - adjustment + value;
     if (offset < from) {
-      throw new IndexOutOfBoundsException();
+      throw new IndexOutOfBoundsException("Negative index provided");
     }
     return offset;
   }