Upkeep: make the error more informative when no matched files with _manual suffix are found

[MonikaFu]

Currently the error says

Maybe consider adding additional i point with something like i Have you inspected the matched files manually after running the match_loanbooks()` function? See the cookboook (link) for explanation of this step.

Right now it says to check the directory parameter if it is correct but there is also a big chance that the user missed the manual step and then the info that no files are found is confusing and the pattern that is expected is not entirely helpful as the user doesn't know why these files are missing.