Silent freeze if source imports non-existant module

If `browserify` is given the `watchify` plugin, it will silently freeze if there's an import of a non-existent module anywhere in the source files reachable via `entries`.

I've isolated a minimal complete reproducible example:

src.js:

```javascript
import foobar from 'someNonExistantPackage';
```

gulpfile.js:

```javascript
const gulp = require('gulp');
const watchify = require('watchify');
const browserify = require('browserify');
const vinylSource = require('vinyl-source-stream');
const vinylBuffer = require('vinyl-buffer');

function createBundler() {
return browserify({
entries: ['./src.js'],
}).plugin(watchify);
}

function bundle(bundler) {
return bundler.bundle()
.pipe(vinylSource('dst.js'))
.pipe(vinylBuffer())
.pipe(gulp.dest('./'))
.on('error', console.error)
.on('end', () => { console.log('Done'); });
}

gulp.task('default', () => {
return bundle(createBundler());
});
```

What I expect to happen is for an error to be thrown. I should see it either in `.on('error', console.error)` or by the gulp task crashing. What happens in practice is that the task freezes without printing anything.

If the `watchify` plugin is removed from the chain, then the task continues and errors successfully:

> ParseError: 'import' and 'export' may appear only with 'sourceType: module'

which is expected, because I removed the babel transform from the example to keep it minimal. If babel is added, then `dst.js` is successfully generated as long as `watchify` is not used.

This happens to me on:

* _browserify_ version 16.5.0
* _watchify_ version 3.11.1

Reproduce the freeze with the minimal src.js import and gulpfile.js using browserify with the watchify plugin. Start at the bundle task and its error/end handlers, then trace how the missing module is handled; done means the task reports an error instead of silently freezing.

Written by the indexing model from the issue text.