cmd: sf: Print error on test failure

The sf test command is used to test spi flashes (and spi masters). Printing
the exact error code is very helpful to those debugging the spi stack.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>