rose edit: fix custom macro status bar reporting

This fixes a problem in `rose edit` where the status bar erroneously reports errors from custom macros.

The problem can be replicated by running `rose edit` on the `demo/rose-config-edit/demo_meta/app/05-validate` app, removing the environment variables, and clicking the "Check fail-if, warn-if, and run all custom validator macros" toolbar button. Although no errors will be reported by the individual macros on the command line, the summary of the individual macros will count a non-zero amount of errors.

@arjclark, please review.