Die gracefully or print when database is not available

[jerome]

When the cupspykota backend runs, we know that we need read+write access to the database. Introduce a norwdb directive which allows either print or fail (and possibly ignore which is fail with an exit(0)), as values in pykota.conf. This directive will let PyKota if it has to fail (as it does actually), or print anyway if the database isn't available in read+write mode.