function jstGetTemplateOrDie(name, opt_loadHtmlFn) {
  var x = jstGetTemplate(name, opt_loadHtmlFn);
  check(x !== null);
  return /** @type Element */(x);
}
This function is the same as 'jstGetTemplate' but, if the template
does not exist, throw an exception.

@param {string} name The ID of the HTML element used as template.
@param {Function} opt_loadHtmlFn A function which, when called, will return
  HTML that contains an element whose ID is 'name'.

@return {Element} The DOM node of the template. (Only element nodes
can be found by ID, hence it's a Element.)