bookmarks: Check if the bookmark to delete exists on the remote

hgext/bookmarks.py

@@ -442,9 +442,12 b' def push(oldpush, ui, repo, dest=None, *'
             if b in repo._bookmarks:
                 ui.status(_("exporting bookmark %s\n") % b)
                 new = repo[b].hex()
-            else:
+            elif b in rb:
                 ui.status(_("deleting remote bookmark %s\n") % b)
                 new = '' # delete
+            else:
+                ui.warn(_('bookmark %s does not exist on the local or remote repository!\n') % b)
+                return 2
             old = rb.get(b, '')
             r = other.pushkey('bookmarks', b, old, new)
             if not r: