Fix logging of crashed finalizers

Previously some finalizers would crash and silently swallow errors if
the finalizer task was terminated, e.g.:

https://groups.google.com/forum/#!topic/celluloid-ruby/ilFEPPpmeZM

This corrects the logging discrepancy and ensures that finalizer crashes
are always logged.
Merge pull request #320 from celluloid/better-finalizer-logging

Fix logging of crashed finalizers