make FP not freak out if required contracts not yet added to system

When FP is added to a site and then deployed, the code is out of sync with the database content (unless some bootstrapping code is added, the contracts referenced in calls to `fine_print_get_signatures` will not yet exist).  To give developers time to add them through the UI after the integration of FP has been deployed, this PR adds one check to ignore unsigned contracts that are not yet published in the system.