bibdocfile: re-add BibRecDocs.check_file_exists
- In f8a1ec18882f0f438b08fd43e93c7ee9b46ef697 the check_file_exists method has been moved from the BibRecDocs class to the BibDoc one. This broke the backward compatibility of the bibdocfile API. Here is a new implementation of the BibRecDocs.check_file_exists method recursively based on the BibDoc.check_file_exists one.