Warnings when cache directory is missing repository.txt

If a cache item is missing a repository.txt, following warnings are printed:

Warning: Cannot open "/home/<username/.cache/qt-unified-linux-online/<sha>/repository.txt" for reading: "No such file or directory"

In case the cache does not contain the repository.txt, the cache item should be deleted and the item refetched. Now the error persists until user manually deletes the cache.

Also the repository.txt should always be there, so check if there are cases where it might not be generated correctly (qbsp file, temporary repository etc.)