Added tests and docs for exception handling

I've added tests and docs for the eval exception handling that you merged yesterday.

Turns out that the finally() method behaves a lot like the done() method in Q https://github.com/kriskowal/q/wiki/API-Reference#promisedoneonfulfilled-onrejected-onprogress 

(Also, t/lib/noEV to prevent AnyEvent and Mojo from loading EV automatically, so we're actually testing the right event loop.)