sql_gen.py: Added reraise_exc

schemas/py_functions.sql: _date(): Raise (or pass through) ValueErrors directly instead of wrapping them in FormatExceptions, to simplify the code. This will also enable later translation of ValueErrors to data_exceptions. When year is required and missing, output a parsable 'null value in column year violates not-null constraint' error.

sql_io.py: put_table(): log_exc(): Handle infinite loops from repeated exceptions by removing all rows, instead of just aborting with a failed assertion

sql_io.py: put_table(): is_function: Fixed bug where special case for unrecoverable errors needed to avoid creating an empty output pkeys table because function mode defines the returned pkeys table separately

sql_io.py: put_table(): is_function: Factored defining the error handling wrapper function out of the main loop because it only needs to run once. Don't log "Trying to insert new rows" in function mode because it's inaccurate.

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