Wrapped Failure Message in CDATA

I think it's wise to wrap the message in `CDATA` since there's a good chance the message will contain XML-like tags.

In my case it was due to one of my failing Angular tests:

```
Expected { $id : '00W', $$childTail : null, $$childHead : null, $$prevSibling : null, $$nextSibling : { $id : '00X', this : <circular reference: Object>, $$listeners : {  }, $$listenerCount : {  }, $parent : { $id : '00V', this : <circular reference: Object>...
```

*(Truncated for sake of sanity)*

In the case of the `<circular reference: Object>`, it was causing the Jenkins job to reject the JUnit XML because it renders an invalid XML document.

With the `CDATA` in place all HTML entities will be nicely escaped, and Jenkins will be happy once again! :+1: