Warn user they didn't give an argument

Here we see it took the user several minutes before finally debugging the problem. ``` $ history 16:07:55 pandoc --print-default-data-file= translations/zh-Hant.yaml 16:08:05 pandoc --print-default-data-file= translation/zh-Hant.yaml 16:09:52 pandoc --print-default-data-file= translation/zh-TW.yaml 16:10:06 pandoc --print-default-data-file=translations/zh-Hant.yaml ``` This might be improved by having the command emit a usage message, > Usage: ...file=FILE instead of always just > Could not find data file