function onExists( error, bool ) {
		var j = i + 1;
		if ( error ) {
			debug( 'Encountered an error when testing for add-on: %s (%d of %d). Error: %s', files[ i ], j, total, error.message );
		} else if ( bool ) {
			debug( 'Detected add-on.' );
			out.push( dirname( files[ i ] ) );
		} else {
			debug( 'Unable to detect add-on.' );
		}
		return done();
	}
Callback invoked upon testing if a file exists.

@private
@param {(Error|null)} error - error object
@param {boolean} bool - boolean indicating whether a file exists
@returns {void}