sql.py: put_table(): Assert that mapping is non-empty

sql.py: mk_select(): Assert that fields list is non-empty

sql.py: DbConn.DbCursor.execute(): Set _is_insert only if query starts with INSERT, so that function definitions containing INSERT are not cached as INSERT statements (exceptions only) themselves

sql.py: DbConn.DbCursor.execute(): Fixed bug where params == None would apparently turn off the mogrifier completely, causing "%"s to be excessively escaped, by just setting params to None if it was [] or () and not using strings.esc_for_mogrify() at all

sql.py: DbConn.DbCursor.execute(): If not using params, escape the query using strings.esc_for_mogrify() in case any literals contained "%"s

strings.py: Added esc_for_mogrify()

sql.py: create_table(): Add indexes on all non-pkey columns, unless turned off or deferred using new param col_indexes

csv2db: Add column indexes on errors table. Use typed_cols and `.to_Col()` to iterate over columns to add indexes on, for the main and errors tables.

sql.py: Added track_data_error(). put_table(): ignore(): Take extra e param for the exception. Use track_data_error() to store the invalid value in the errors table.

sql_gen.py: Join.to_str(): Add newline before and after right table if it's been renamed (and therefore takes up multiple lines)