Fail if -config explicitly given on command line and given file has problems

At the moment, the programs in pto3-go use some sort of default config if the -config option is given on the command line but the referenced file does not exist.

Replicate, e.g., with ptonorm -config sajdgas ...

It would be better not to fall back on a default config and instead to fail with an error message if -config is given on the command line and if there is a problem with that config file.