code: @Nonnull
      public Token peek(String message) throws IOException {
          if (!hasNext()) {
              throw failure(lineNo, linePos + 1, 0, "Expected %s: Got end of file", message);
          }
          return unreadToken;
      }
docs: Return the next token or throw an exception. Though it does not consume
  that token.
  
  @param message Message to add to exception if there are no more JSON
  tokens on the stream.
  @return The next token.
  @throws IOException If unable to read from stream.