improve error handling in bucketExists()

Added call of validate() in bucketExists(). Reason: if the request for
checking whether a bucket exists contains invalid credentials (see unit
test), then S3 does not return an XML response. Instead the HTTP
response body is empty. This in turn caused the validate() function
to throw an XML parsing exception which in turn shadowed the original
error. This behavior is different compared to when we list buckets. In
this case, the response contains an HTTP body with a detailed error
message encoded as XML.