mgmt/mcumgr: Fix image erase returning improper error code on error
mgmt/mcumgr: Fix image erase returning improper error code on error

The img_mgmt_erase could return MGMT_ERR_EOK even when
img_mgmt_impl_erase_slot failed.

Fixes #50522

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr