Consider warning when an active bookmark name is not found in a reloaded bookmark file

[shipmints]

The cases I've experienced are:

1. Renaming a bufferlo bookmark in a second Emacs session on the same host, and then reloading the newly changed bookmark file in the first session.
2. Deleting a bookmark in the same way.

We should be trivially be able to provide a warning in any active Emacs which reloaded the now-changed bookmark file. I think we already have some code that can scan "set" bookmarks for invalid constituents, and that could also be run to provide a warning when constituent names are now no longer valid.

[shipmints]

bookmark.el doesn't have a hook for bookmark file "after load" (I suppose I could submit a patch for that). It could suffice to warn when saving an active bufferlo bookmark that is not present in the in-memory bookmark list.