deposit: accordion does not pick up nested errors

Package version (if known): v4.10.1

## Describe the bug
In `AccordionField.js` nested errors that are not objects with `message` and `severity` keys are not collected. While the FormFeedback component lists errors in a respective section, its accordion header is not aware of the contained errors.

## Steps to Reproduce
1. Go to 'New upload' in the demo instance
2. Scroll down to e.g. the Alternate identifiers section
3. Select a Scheme but do not provide an identifier
4. Hit 'Save draft'
5. Expect the error to be indicated in the Form Feedback banner
6. **Expect the accordion to be highlighted (it is not)**

## Screenshots (if applicable)

## Additional context