[Forwardport to 4.1] - Transparent error feedback for custom page copy actions is broken

The CMS 11.0 channel manager translates error responses from the back-end into localised, user-friendly error messages (feedback toast). The new mechanism breaks the 10.2-possibility to provide custom error messages in case some custom error occurred during a page-copy action (see [1]).

This should be fixed before GA (and merged to master): Document that a new ClientError.CUSTOM_ERROR should be used in the back-end, which, when received by the front-end, bypasses the error dispatching logic and displays the provided custom error message transparently.

This requires changes to HST, the Channel Manager and the Documentation.

[1] https://www.onehippo.org/library/concepts/template-composer/hook-into-page-copy-process.html