cmd: sf: Display errno on erase failure

If there is an error while erasing SPI flash, no errno is displayed. This
makes it difficult to determine the cause of the error. This change mirrors
the logic for write errors above.

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>