Avoid a crash when saving a new bookmark and elpher-bookarks-file dir is missing

As the ‘elpher-bookmarks-file’ is customizable, one can set it to a
non-existent dir (like ‘~/.emacs.d/etc/elpher/bookrmarks’). This commit just
check that the directory part of ‘elpher-bookrmarks-file’ exists before trying
to save it.