scripts: runners: improve hex file detection and output

This is a band-aid to make it more obvious to potential users of 'west
sign' and 'west flash' which hex file they are flashing, when they are
falling back on a binary file, and erroring out when a hex file does
not exist.

Fixes: #18201
Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>