Fail test when targeted element does not exist

Fixes #12.

This fix tests for the presence of the word `failed` in the output from `PhantomCSS`. The context in which it can appear is when the screenshot capture fails e.g.:

```
[PhantomCSS] Screenshot capture failed:  No element matching selector found: #foo
```

Thus, the respective line is pushed into the errors array and the `failCount` incremented. This ensures that the test is marked as failed and a non-zero exit code will be returned.

I've also removed all trailing whitespace.

Any feedback is appreciated.