cmd: fdt: Print error message when fdt application fails

There are lots of reason why a FDT application might fail, the
error code might give an indication. Let the error code translate
in a error string so users can try to understand what went wrong.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Simon Glass <sjg@chromium.org>