code: function(callback) {
          grunt.log.writeln('Prepare tmp path...');
          if (!grunt.file.isDir(tmpPath)) {
            if (grunt.file.exists(tmpPath)) {
              callback(new Error(util.format('"%s" is not a directory', tmpPath)));
            }
            else {
              grunt.file.mkdir(tmpPath);
              callback(null);
            }
          }
          else {
            callback(null);
          }
        }
docs: make sure tmpPath exists