Zephyr MCUboot: Avoid relying on assert statements and add explicit return code checks

[namjoshiniks]

There some areas in the code where there are assert statements and no explicit return code checks. We should make sure we do explicit checks and return on errors

For e.g.

https://github.com/mcu-tools/mcuboot/blob/main/boot/zephyr/single_loader.c#L192
https://github.com/mcu-tools/mcuboot/blob/main/boot/zephyr/firmware_loader.c#L118