access to buckets owned by other users not possible with ceph radosgw

[jrandall]

Using minio-go, I cannot access buckets owned by other users (with appropriate ACLs set) on a Ceph radosgw. It is expected that they cannot be listed, but testing for existence or accessing them by name should work.

I have identified the underlying issue and will submit a PR shortly.

The problem is that Ceph radosgw is not returning a <Message> in the AccessDenied error response, but processBucketLocationResponse is checking that an Access Denied string is contained in the message (https://github.com/minio/minio-go/blob/master/bucket-cache.go#L128).

An example error response I get during a BucketExists call on an accessible bucket owned by another user is:

<?xml version="1.0" encoding="UTF-8"?><Error><Code>AccessDenied</Code><BucketName>npg-cloud-realign-wip</BucketName><RequestId>tx0000000000000002b1dad-0058d25cfa-6ccee7-default</RequestId><HostId>6ccee7-default-default</HostId></Error>

Because <Message> is not included at all in the XML, it will be left at the nil value of "" in the errResp ErrorResponse structure. My PR will test for an empty Message and treat that the same way as a message containing the string "Access Denied".