sql_gen.py: Exceptions: Added suppress_exc and use it in ExcHandler.to_str()

README.TXT: Backups: After a new import: Added step to delete previous imports so they won't bloat the full DB backup. (Note that these imports have already been backed up, and only the most recent import needs to be live in the DB.)

README.TXT: Backups: Documented what to do after a new import

backups/Makefile: Full DB: Added vegbien.backup/all to run both test and rotate

README.TXT: Renamed Maintenance section to Backups for clarity

backups/Makefile: .sql: When testing, turn it off so make won't skip `.sql: %` in favor of it

backups/Makefile: Split %.backup and %.sql into separate targets for clarity

inputs/import.stats.xls: Updated with stats from latest import. Note that this import adds data provider feedback for SQL functions as well as additional date processing using _date().

schemas/py_functions.sql: _date(): Re-enabled now that exceptions thrown are properly handled. FormatException: Support raising parsable data_exceptions when provided with the value that was invalid. Date parsing mode: Return date as the value in FormatException so it can be filtered out automatically by column-based import.

sql_io.py: put_table(): is_function: Creating error handling wrapper function: Fixed bug where needed to cast NULL returned in error handler to appropriate type, because it's contained within a SELECT query which does not do implicit casts from type unknown