Error out if user provides errorneous format specification

Continuation to

- https://github.com/con/duct/pull/183#issuecomment-2383988409

and a brief discussion on matrix. I think it is bad to be "too tolerant" to user mis-specifications, in the end it would only delay the pains and possibly make it harder to figure out what to fix later on. You can generally avoid throwing traceback (unless e.g. some `DUCT_DEVEL` envvar set), but should exit with non-0 so user detects error right away and fixes for it.