Altered ``get_bucket`` to use ``HEAD``.

This is mostly backward-compatible (& less expensive). However, if you were parsing exception messages from this method, you should refer to the documentation on how to change your code.
Updated the wording to stress that thousands of
keys weren't being fetched before.
Merge branch 's3-get-bucket-fix' into develop
Fixes #2082, #2078.
Fixed a test failure in Route53.