com_item_missingness: Confusing Error message

For the com_item_missingness function, the error message on ill-formatted missing tables in argument cause_label_df is quite confusing:

replacement has 0 rows, data has 138

The missing table should be verified regarding its column names and types for being valid. Also, a duplicate check should be amended.

Later, the table could be extended by another column referring variable names -- if given, we could have the same missing code with different meanings for different variables.