PSOC6: Fix post build script
PSOC6: Fix post build script

Raise an exception in case of a failure to find an image to use
for the binary signature. This prevents the method from assuming
the image is always successfully retrieved and crash when
attempting to print a message

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed