join: In error message when map column names don't match, include the actual column names

Makefiles: Added .DELETE_ON_ERROR to delete target if recipe fails

VegBIEN mappings: plantnames: Nest taxons hierarchically using plantname.parent_id. Mappings using _forEach: Append a "," to the `in` list so that mappings will sort from shortest to longest `in` list ("]" comes after "," in ASCII, causing this not to happen without the trailing ",").

xpath.py: parse(): _paths(): Remove trailing ","

xpath_func.py: _forEach: Made syntax more natural-looking by using values instead of names for string args and attrs instead of branches for array args

xpath.py: parse() Fixed bug in _paths() where empty lists would be parsed as a list containing a single empty path, instead of as an empty list

VegBIEN mappings: Place names: Use _forEach to simplify XPaths for recursively nested places

bin/map: In debug mode, print output XPaths

xpath_func.py: _forEach: Fixed to support _val replacements anywhere, by doing a string-based search-and-replace on a quoted XPath instead of a list-based search-and-replace on an already-parsed XPath

xpath_func.py: Renamed _for to _forEach. Finished implementing _forEach.