Return error from app and command actions. See #66.

This introduces a significant breaking change, and I don't expect it to
be merged off-hand. I do think that it's worth discussion, since it
seems like a very idiomatic choice in terms of how errors would be handled.

A similar backwards-incompatible change was introduced in e6e64114,
allowing the app's Run method to return an error.
Fix example code
merge