code: function(error, engine) {
  
      if (error.message.indexOf('ENOENT') > -1) {
        grunt.log.error(error.message);
  
        grunt.fail.warn('\nPlease ensure ' + GFX_ENGINES[engine].name + ' is installed correctly.\n' +
        '`brew install ' + GFX_ENGINES[engine].brewurl + '` or see ' + GFX_ENGINES[engine].url + ' for more details.\n' +
        'Alternatively, set options.engine to \'' + GFX_ENGINES[engine].alternative.code + '\' to use ' + GFX_ENGINES[engine].alternative.name + '.\n');
      } else {
        grunt.fail.warn(error.message);
      }
    }
docs: Handle showing errors to the user.
  
  @private
  @param   {string}          error     The error message.
  @param   {string}          engine    The graphics engine being used.