[storage-local] Returns 500 when moving dir over a file

Error received:

`{"RID":"api-23425705-61c8-4e64-8984-cc02116f8231","level":"error","msg":"Cannot rename resource: [map[err:rename /home/clawiod/data/notmp/fileauth/demo/photos.bak /home/clawiod/data/notmp/fileauth/demo/passwd: not a directory]]","time":"2015-09-13T14:28:11Z"}`

We have to catch `not a directory` and return other error, maybe Precondition failed`
