Custom error message for attachment validation in FileUploadWithTag

### Description

Provide the ability to customize the error message when the edit window is open but the user hasn't selected a tag or clicked on save before proceeding to the next step. This functionality can be implemented in the EditWindowComponent file.

https://altinn.slack.com/archives/C02FK32SFA4/p1691396971192909

### Additional Information

_No response_

Start by reading EditWindowComponent and tracing how FileUploadWithTag reports validation when no tag is selected or the user proceeds without saving. Determine the existing error-message path and how customization is configured. Done means the attachment validation message can be customized and the default behavior remains covered by the relevant tests.

Written by the indexing model from the issue text.