coverage report generation fails: mocha > gulp-mocha > gulp-istanbul

Question

I'm using gulp-mocha and gulp-istanbul to pipe mocha run through to istanbul for coverage collection.

I'm using done(error) to denote a test fail in mocha test cases.

When all tests pass, istanbul runs coverage collection and produces reports. When, any test fails, instanbul fails to execute.

gulp.task('cov', ['pre-cov'], function () {
  return gulp
    .src(['test/service/*.js'])
    .pipe(mocha({reporter: 'JSON'})
      .once('error', () => {
        this.emit('end');  
      })
      .once('end', () => {
        this.emit('end');  
      }))
    .pipe(istanbul.writeReports({
      dir: 'tmp/unit-test-coverage',
      reporters: [ 'lcov','text','text-summary'],
      reportOpts: { dir: 'tmp/unit-test-coverage'}
    }))
    .pipe(istanbul.enforceThresholds({ 
      thresholds: { global: 10 } 
    }))
    .on('end', () => {console.log("EXIT");process.exit(1);});
});

I am however emitting an end event to gulp when there is a mocha test fail/error. But even that is failing to execute the on(end) callback at gulp.

Seems, I am missing something here, which would allow me to continue onward to the next stage, which I am unable to figure out with mocha or istanbul docs.

-- thanks for the help in advance.