0.9.0: building PDF files requires a classlist

The paper pdf builder tries uncond to get the classlist, even if all paper_number columns are blank.

That's a regression: it was very common in 0.8.x to sit the test before uploading a classlist (when using non-prenamed papers obviously).

I guess we should try: except: the classlist getter in the paper builder code. Possibly its enough to set classlist = [] in the except: block. Fix this after !1464 (merged) which gives us a specific exception to test for: PlomNoClasslist, a subclass of PlomServerNotReady.