cubicweb #344306 enhance the input of multiple entities [open]

Some entities require inputing 1 or more linked entities of another kind (in the expense application, an Expense has at least one ExpenseLine, and possibly more, for instance).

When inputing several such linked entities, if a validation problem occurs when the user presses the apply or validate button, the faulty field is not highlighted, making it difficult to find, especially if the user has typed in a large number of linked entities.

Example error message:

paid_for: L'entité #3952 de type ExpenseLine doit nécessairement être reliée à une
autre via la relation paid_for

Problems with the current implementation:

• the problematic field should be visually highlighted (red background for instance)
• the error message gives an eid for the faulty linked entity, but this eid is not displayed anywhere in the edition field (because the entities do not exist yet : they only have an order number). The message could use that number to make things easier.
• translation issues for entities and relations (reported separately)