Don't try to access `stack` of undefined exceptions.

Currently, throwing `undefined` will cause Jest to fatal: `TypeError: Cannot read property 'stack' of undefined`.
I'm not sure. With an empty string or — I imagine — any falsey value, Jest prints:

```
● Jasmine › it should not fatal on undefined exceptions
  - No stack trace found
1/1 tests failed
```

With a string like `No stack trace found`, Jest prints:

```
● Jasmine › it should not fatal on undefined exceptions
  - Exception
1/1 tests failed
```

You can't tell, but `Exception` is also red and underlined. Would someone using Jest really grep the Jest codebase for `'No stack trace found'` in the event of an error in their test?