build_papers: fails with KeyError if not enough rows in the database

1. Add classlit and spec but don't make db.
2. call build_papers. Note: not directly possible via command line tools at the moment as it will automatically build DB first.

Traceback (most recent call last):
  File "/home/cbm/src/plom/plom.git/plom/manager/manager.py", line 724, in buildPapers
    build_papers(
  File "/home/cbm/src/plom/plom.git/plom/create/start_messenger.py", line 48, in wrapped
    return f(*args, **kwargs)
  File "/home/cbm/src/plom/plom.git/plom/create/buildDatabaseAndPapers.py", line 104, in build_papers
    build_papers_backend(
  File "/home/cbm/src/plom/plom.git/plom/create/buildNamedPDF.py", line 111, in build_papers_backend
    question_version_map = global_question_version_map[paper_index]
KeyError: 1

In this case the qvmap is empty. Should be an error but likely could be a better error than this.