servers: xtfs_stat returns values != 0 if there is an error (file not found,...

servers: xtfs_stat returns values != 0 if there is an error (file not found, not an xtreemfs volume...)

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@1617 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534