Distinguish between errors and failures in webapptests

[jernst]

If I expect a given function to return 5 and:

1. it returns 5, that's a "success"
2. it returns 6, that's a "failure".
3. it blows up, that's an "error"

Webapptest currently does not distinguish (well) between 2 and 3, and should. See #36 (comment)