fix callback chaining benchmarking example

if there was an error in any query, callbacks_full_pipeline.py would
hang because "self.event" (or "finished_event" in the docs) would never
get set. this also fixes some minor pep8 issues.
whoops. fix that typo.
fix it there too.
Merge pull request #77 from gregbanks/fix-broken-callback-chaining-example

Fix broken callback chaining example