Cleanup chunks only when rescuing from an error [Fixes #7]

Relying on the $! variable to made it possible to unintentionally
destroy chunks during a successful pass if the $! variable was set by
something else upstream.

E.g. $! is set to an EOFError under the Pow server, because the Nack
server runs everything through an (expected) EOFError rescue block:

https://github.com/josh/nack/blob/master/lib/nack/server.rb#L98-L101