Bug: getLibraries() not working when there is a missing library
scenario to reproduce:
 
add a sketch file as library, in finder move that library to a different folder.

Run the following:
```
var libraries = require('sketch/dom').getLibraries();
console.log(libraries);
```

output is:
```
TypeError: null is not an object (evaluating 'e.timeIntervalSince1970')
line: 1
sourceURL: /Applications/Sketch.app/Contents/Resources/SketchAPI_dom.js
column: 64745
```

