Document exceptions (currently) raised from 'Bucket.delete'.

Note that #164 would change this (by adding fine-grained exceptions
for 409, 401, etc.).

Fixes #159 (but should be revisited if #164 lands).
Merge pull request #321 from tseaver/159-document_exceptions_for_bucket_delete

Fix #159: Document exceptions (currently) raised from 'Bucket.delete'.