scripts: west sign: improve user friendliness
scripts: west sign: improve user friendliness

Add more error handling and warnings. Doing this nicely requires a bit
of re-work to the control flow.

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

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