* bookmark.el (bookmark-handle-bookmark): If bookmark has no file,
don't do anything related to relocating, just return nil.
(bookmark-error-no-filename): New error.
(bookmark-default-handler): Signal `bookmark-error-no-filename' if
bookmark has no file. Don't even attempt to handle things that
are not files; the whole point of custom handlers is to keep that
knowledge elsewhere anyway. Tighten some comments.
(bookmark-file-or-variation-thereof): Remove now-unused function.
(bookmark-location): Doc string fix.