Throw exception if non-sitetree parent CMSEditLink is empty

[emteknetnz]

Follow on from #1406 (comment)

If a non-inline editable elements non site tree parent has an empty CMSEditLink, then the form for the edit form cannot be created.

The current behaviour is the user will be redirected to the home page on the frontend. Instead, a hard exception should be thrown.

PRs

• ENH Throw exception if empty parent CMSEditLink #1408
• DOC Throw exception if empty parent CMSEditLink developer-docs#871