vfs: Make error messages when mounting rootfs more verbose.

Provide a better error message instead of simply printing the error
codes.

Before:

  failed to create /dev, error = 17

After:

  failed to create /dev, error = File exists

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>