function onRender( error, html ) {
			if ( error ) {
				debug( 'Encountered an error when rendering file. File: %s (%d of %d). Error: %s', files[ i ].file, i+1, total, error.message );
				return clbk( error );
			}
			debug( 'Rendered file.' );
			files[ i ].data = html;

			nextTick( done );
		}
Callback invoked after rendering a file.

@private
@param {(Error|null)} error - error object
@param {string} html - rendered HTML
@returns {void}