Require a more strict input format for specifying Project Gutenberg works

[tanius]

(This issue does not have to be solved right now. Only recording it here for now. We'll solve it when we need this script again.)

Currently, the script to extract Project Gutenberg metadata will interpret all numbers as specifying Project Gutenberg works. This can lead to errors, as some numbers might be year numbers etc..

Example: in the following line, the current script would interpret both "1920" and "4924" as numbers specifying Project Gutenberg works, and proceed by extracting metadata for them.

Dry-Farming. Published 1920. Project Gutenberg text no. 4924.

Solution proposal: only accept input lines that contain a single number and nothing else per line, and ignore lines with anything else present (while emitting a warning for such lines).