Add check for author metadata to the 'traffic lights' alert

[rupertgatti]

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This title is listed without the author fields being complete:
https://thoth.pub/books/2b6cd7bb-cd95-4e11-9199-bfae52304adb

but the traffic light warning show only missing urls for the pdf:

Untitled-2.tif

[ja573]

We don't check all constraints and provide a report of why that file cannot be generated, we just return the first error we encounter