Convert the file storage rpcs to new style result

This patch converts all three file-storage rpc (create, delete, rename)
to new style result. This is done in a single patch as they all use a
helper function which itself needs to/can be converted.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>