callback after emit error

if use gulp-plumber it will stop watching because halt from emit error.

the condition code is like below
```
var plumber = require('gulp-plumber');
gulp.task('build', function(){
  gulp.src('*.styl')
    .pipe(plumber())
    .pipe(gulp-stylus())
    .pipe(gulp-dest('.'))
});

gulp.task('watch', function(){
  gulp.watch('*.style', 'build')
});
```