Allow Errors on the group level

[marstamm]

Is your feature request related to a problem? Please describe

With our approach to end-to-end validation, we try to hint users to problems in the model in any potential place, so the user won't miss any problem. Problems related to whole sections (such as using extension properties prior to 8.1) can not be displayed properly, resulting in redundant highlighting on a per-item basis (see camunda/linting#14).

Describe the solution you'd like

In case the problem is with the whole section (such as with the extension properties example), an optional error message should be shown in the section header.

Describe alternatives you've considered

Additional context

Created as follow-up to #180

[philippfromme]

Putting this in the backlog for now.