Its fairly common to build papers without yet having a classlist. Fix a regression whereby the server needed to have a classlist even if no one was prenamed. Fixes #2148 (closed).
build_papers: throw error if no database
This function claimed it raises PlomConflict but it did not. Likely it
did before the latest changes to prenaming (before the
column changes). Fixes #2150 (closed).
few other little fixes