Document exceptions (currently) raised from 'Connection.create_bucket'.

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

Fixes #166 (but should be revisited if #164 lands).
Merge pull request #322 from tseaver/166-document_exceptions_for_connection_create_bucket

Fix #166: Document exceptions (currently) raised from 'Connection.create_bucket'.