Document how 'NotFoundError' propagates.

Describe suppressing it via 'bucket.delete_keys([key], on_error=lambda k:None'.

Fixes #160.
Typo:  no star-arg required.
Note source of exception.

Incorporates feedback from @dhermes.
Merge pull request #328 from tseaver/160-doc_avoid_notfound_when_del_key

Fix #160:  document how 'NotFoundError' propagates.