rose edit: fix error menu.

This fixes errors in variable widget menu construction when
there is an error for that variable created by a custom macro.

The problem was caused by a '/' in the custom macro name, which
interfered with the err_item lookup.

To replicate the problem, load the `demo/rose-config-edit/demo_meta/app/05-validate` app, run all validator macros, navigate to `env` and try to open one of the variable menus. Without this fix, there will be a traceback. With the fix, the menu should display correctly.

@arjclark, please review.