Stop process if master_key_* is missing from prod.keys

[msdos]

Suppose you're with prod.keys extracted with a 16.x.x firmware (or older) and you're trying to convert a 17.x.x nsp.

Although an error message is shown:

master_key_10 missing from <user_path>/prod.keys! This will lead to corrupted output.

Instead of aborting the process it continues giving error tracebacks. IMHO if a master key is not found, it should show the message above but aborting the process with a sys.exit(1).