uncommented line that sets default prefix, added an error message for when fmd lookup fails

hi,

I noticed the code was failing to return correct file info, so I put some prints in to try to figure it out:

180716 03:23:11 ^[[49;31mERROR^[[0m S3Store:406                    filepath=/eos/test/testbuckettestfile errno=2 msg="File does not exist"

for reference, this is the query:

davix-ls  --s3accesskey daemon --s3secretkey supersecretkey --s3alternate http://mgm-master.testnet:8000/testbucket/

uncommenting this line seems to fix it? https://gitlab.cern.ch/dss/eos/blob/dev/mgm/http/s3/S3Store.cc#L274

<?xml version="1.0" encoding="UTF-8"?><ListBucketResult xmlns="http://doc.s3.amazonaws.com/2006-03-01"><Name>testbucket</Name><Prefix/><Marker/><Delimiter>/</Delimiter><MaxKeys>10000</MaxKeys><IsTruncated>false</IsTruncated><Contents><Key>/testfile</Key><LastModified>2018-07-16T03:44:23Z</LastModified><ETag>b6d81b360a5672d80c27430f39153e2c</ETag><Size>1048576</Size><StorageClass>STANDARD</StorageClass><Owner><ID>daemon</ID><DisplayName>daemon:daemon</DisplayName></Owner></Contents></ListBucketResult>