Template not found error

I ran into a  `Fatal error: baseClass is not defined` message when trying to use a template file with the `webfont` task. It took me an embarrassing amount of time to realize that I configured the wrong path to my template file.

This pull request handles invalid template paths with a more descriptive `Fatal error: Cannot find template at path: not/a/path/to/template.css` message.

I attempted to test this behavior, but I couldn't figure out how to get nodeunit to catch the error thrown by `grunt.task.run`. So instead, I included a [commit](https://github.com/eschwartz/grunt-webfont/commit/9de515fc6467cf7373fb225c8e7dcc6f03265df0) which demonstrates the previous behavior.