custom fields: error is not displayed after saving a few times

[jrcastro2]

Package version (if known):

Describe the bug

When introducing a wrong value in one of the custom fields, after the first save, the error is displayed but after saving it again it the error will disappear, although the value is still incorrect in the form

Steps to Reproduce

1. Go to 'New upload'
2. Fill in required fields
3. Fill in one of the custom fields with a invalid value
4. Save
5. Modify any other field except for the custom field with the wrong value
6. Save
7. Save again
8. Error disappeared

Expected behavior

Error should be displayed as long as the value is wrong in the form.

[ppanero]

There are two possible solutions:

• Frontend: Keep the current state in redux, then do a merge between the current content and the one returned by the REST API.
• Backend: Save invalid data in drafts so it is a rest api driven implementation (personally, I prefer this option).
  That will require a merge between what is produced by the marshmallow schema and the input data.