Missing articles – KeyError: 'filename'

Some articles are missing, for example:
```
ERROR: Could not process ./2006-08-09_co-prinesl-podzim.md

  | KeyError: 'filename'

```
This appears to be due to this change in Danube Delta: https://github.com/honzajavorek/danube-delta/commit/5ede6efd0bf9f5d2b7a02d513bfb184fa627504b

What worries me is that the build succeeded with missing articles.