We import "github.com/pkg/errors" in the code, and actually put
the "github.com/pingcap/errors" there in the vendor path.
That's a dirty hack, and prevent the introduce of Go module.
DESC table_with_generated_column;
SHOW CREATE TABLE table_with_generated_column;
If generated expression in DDL references bad column, throw error;
If alter table change/drop column dependent by other columns, throw error;
if alter table change/modify generated columns, throw error if needs.
