Disable CURLOPT_FAILONERROR for CheckBucket

curl will not consume the body of a response when CURLOPT_FAILONERROR
is set.  This prevents logging of responses for failed requests.
Emit response on failed CheckBucket requests

This allows callers to diagnose errors like InvalidAccessKeyId and
RequestTimeTooSkewed.
Merge pull request #60 from andrewgaul/check-bucket-disable-fail-on-error

Emit user-friendly log messages on failed CheckBucket requests