fix (server): uncaught error via fs.stat on symlink

Hey All,

Preface: I've never contributed to large open source projects like `karma`, please bear with me and let me know what I can do to help out.

I was getting this error when running `karma` inside of `/Users/muffs/project` via `./node_modules/.bin/karma start`:
```javascript
{ [Error: ENOENT, stat '/Users/muffs/project/static/js/lib/expect/support/mocha.js']
  errno: 34,
  code: 'ENOENT',
  path: '/Users/muffs/project/static/js/lib/expect/support/mocha.js' } '/Users/muffs/project/static/js/lib/expect/support/mocha.js'
ERROR [karma]: [TypeError: Cannot call method 'isDirectory' of undefined]
TypeError: Cannot call method 'isDirectory' of undefined
    at List.refresh (/Users/muffs/project/node_modules/karma/lib/file-list.js:187:23)
    at Object.oncomplete (fs.js:297:15)
```

Turns out that `/Users/muffs/project/static/js/lib/expect/support/mocha.js` is a symlink to `../node_modules/mocha/mocha.js`. Which is broken in this case. So it seems as though the `refresh` step would break if it found a broken symlink.

My small change ignores errors on fs.stat.