code: @GwtIncompatible("incompatible method")
      public static String getRootCauseMessage(final Throwable th) {
          Throwable root = ExceptionUtils.getRootCause(th);
          root = root == null ? th : root;
          return getMessage(root);
      }
docs: Gets a short message summarising the root cause exception.
  <p>
  The message returned is of the form
  {ClassNameWithoutPackage}: {ThrowableMessage}
  
  @param th  the throwable to get a message for, null returns empty string
  @return the message, non-null
  @since Commons Lang 2.2