largefiles: fix a traceback in lfconvert if a largefile is missing (issue3519) The largefile may be missing for various reasons, including that a remote repository was cloned without the --all-largefiles option. Therefore, it seems reasonable to attempt to download the missing files and failing that, abort and indicate the affected file and revision so the user can manually fix the problem.

bookmarks: further flatten code This hopefully clarifies the behavior when no NAME is passed, by separating the branches for listing bookmarks vs. deactivating the current bookmark.