Crash on wrong book cover filepath

[cuihtlauac]

See PR #3093. Issue that caused the crash (and the revert in PR #3108)

19.05.25 06:34:42.148    dream.logger  INFO REQ 1 GET /books 127.0.0.1:37708 Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36
19.05.25 06:34:42.198    dream.logger  WARN REQ 1 Aborted by: Invalid_argument("'books/using-understanding-and-unraveling-the-ocaml-language.png' is rendered via Media.url, but it is not an media!")
19.05.25 06:34:42.198    dream.logger  WARN Raised at Ocamlorg_static.Media.url in file "src/ocamlorg_static/ocamlorg_static.ml", lines 15-18, characters 6-24
19.05.25 06:34:42.198    dream.logger  WARN Called from Ocamlorg_frontend__Learn_components.book_cover in file "src/ocamlorg_frontend/components/learn_components.eml", line 94, characters 54-1
19.05.25 06:34:42.198    dream.logger  WARN Called from Ocamlorg_frontend__Books.render_book in file "src/ocamlorg_frontend/pages/books.eml", lines 10-13, characters 35-1
19.05.25 06:34:42.198    dream.logger  WARN Called from Ocamlorg_frontend__Books.render.(fun) in file "src/ocamlorg_frontend/pages/books.eml", line 72, characters 35-1
19.05.25 06:34:42.198    dream.logger  WARN Called from Stdlib__List.iter in file "list.ml", line 112, characters 12-15
19.05.25 06:34:42.198    dream.logger  WARN Called from Ocamlorg_frontend__Books.render in file "src/ocamlorg_frontend/pages/books.eml", lines 70-72, characters 15-16
19.05.25 06:34:42.198    dream.logger  WARN Called from Ocamlorg_web__Handler.books in file "src/ocamlorg_web/lib/handler.ml", line 503, characters 4-75
19.05.25 06:34:42.198    dream.logger  WARN Called from Dream_encoding.compress in file "lib/dream_encoding.ml", line 158, characters 21-32
19.05.25 06:34:42.198    dream.logger  WARN Called from Lwt.Sequence_associated_storage.with_value in file "src/core/lwt.ml", line 804, characters 19-23
19.05.25 06:34:42.198    dream.logger  WARN Re-raised at Lwt.Sequence_associated_storage.with_value in file "src/core/lwt.ml", line 809, characters 6-15
19.05.25 06:34:42.198    dream.logger  WARN Called from Lwt.Sequential_composition.try_bind in file "src/core/lwt.ml", line 2139, characters 10-14
19.05.25 06:34:42.198      dream.http ERROR REQ 1 Invalid_argument("'books/using-understanding-and-unraveling-the-ocaml-language.png' is rendered via Media.url, but it is not an media!")
19.05.25 06:34:42.198      dream.http ERROR REQ 1 Raised at Stdlib__Map.Make.find in file "map.ml", line 141, characters 10-25
19.05.25 06:34:42.198      dream.http ERROR REQ 1 Called from Logs.Tag.find in file "src/logs.ml", line 154, characters 14-32