No indication where an error is located in case of a collapsed compound

Since brXM 14 it has been possible to collapse multiple compounds / content blocks in the editor, see CMS-11043.
This is nice, however, if a collapsed compound has an error (e.g. missing mandatory field), there is no visual clue any more where the error is located when saving the document.

Fixing this would be to either:

• indicate at the (collapsed) compound level that it contains an error
• automatically expand a collapsed compound

The last option is the preferred option