Include column name in build_attrmap_by_position's error reports.

Include column name in build_attrmap_by_position's error reports.

Formerly we only provided the column number, but it's frequently
more useful to mention the column name.  The input tupdesc often
doesn't have useful column names, but the output tupdesc usually
contains user-supplied names, so report that one.

Author: Marcos Pegoraro 
Co-authored-by: jian he 
Co-authored-by: Tom Lane 
Co-authored-by: Erik Wienhold 
Reviewed-by: Vladlen Popolitov 
Discussion: https://postgr.es/m/CAB-JLwanky28gjAMdnMh1CjyO1b2zLdr6UOA1-oY9G7PVL9KKQ@mail.gmail.com