Mandatory fields for metadata

[NedPool]

I've also mentioned this in the Sub-Folders thread here, but I'm running into this issue when trying to save metadata.

When clicking the Apply Changes button (either for the folder or for folder contents), it presents a box stating that it cannot save due to missing mandatory fields.

Can this box also state which fields are mandatory (that aren't filled)? Currently there is no indicator that any fields are mandatory.

Thanks for the great work!