flash_img: add error check to 'flash_progressive_erase'
flash_img: add error check to 'flash_progressive_erase'

Currently the error codes returne from this function is ignored.
Add error reporting to allow users to handle what is reported.

Signed-off-by: Håkon Øye Amundsen <4238ca2ac515887d8a25d5e30932b12519303bf7@nordicsemi.no>

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