Add todo: Rewrite libimagerror with chain-error crate

doc/src/09000-todos.md

@@ -76,6 +76,11 @@ Now the modules that are not yet started:
   wrapping store types
 - [ ] Rewrite logger to allow config/env-var based module white/blacklisting and
   writing log to file
+- [ ] Rewrite libimagerror to be based on `chain-error` crate. This basically
+  removes our error infrastructure, though existing traits and convenience
+  functionality should be kept (for example the `MapErrInto` or the `IntoError`
+  traits), as much as possible). If the switch to `chain-error` would require
+  too much code to be rewritten, reconsider.
 
 ## User Interface