Makes unfulfilled dependency error message more helpful

This shows all unfulfilled dependencies in the unfulfilled dependency error message rather than choosing an arbitrary one.

It also removes the overly verbose and distracting previous tasks. In most of my error e-mails I'll have a few hundred previous tasks. This makes it difficult to see which task was actually unfulfilled and provides no useful information. Mentioning a bunch of unrelated tasks also complicates searching my e-mail for errors related to specific tasks. If I really needed to know the workers history, I could just check the logs.