Error out if user provides errorneous format specification

[yarikoptic]

Continuation to

• Add custom formatter conversion flags and colors based on datalad ls #183 (comment)

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.