Add warning when attempting to use element with tab as nested content

[Fjellvang]

In umbraco 8.17 if you create an Element type and add a Tab to it you cannot use it in a nested content picker.

For umbraco 9, we display a nice error and attempt to guide the user to use Block list editor. This warning is not implemented in umbraco 8, and hence it will leave some editors confused.

Could we add this warning to umbraco 8?

Setup In umbraco 8:

the nice warning in 9: