Be nicer if a template is missing

If a template is referenced but not available (the class is missing or renamed), then Template Field throws an exception.

This happens in template_api/includes/plugins.inc:
throw new Exception("$klass cannot be found, trying to load $plugin_key plugin.");

At this point, the code does not have access to the template class or plugin name that was referenced on the content, so you get the less than helpful message: " cannot be found, trying to load plugin."