sd-journal: return error when we cannot open a file

Lack of this caused journalctl not to display a hint about missing groups
properly when the user lacks permissions.

(cherry picked from commit 7b300be75e)

src/journal/sd-journal.c

@@ -1248,7 +1248,7 @@ static int add_file(sd_journal *j, const char *prefix, const char *filename) {
         r = add_any_file(j, path);
         if (r == -ENOENT)
                 return 0;
-        return 0;
+        return r;
 }
 
 static int remove_file(sd_journal *j, const char *prefix, const char *filename) {