Fix EZP-20992: Wrong scope in contentViewGenerateError when using DFS and user-permissions

This patch fixes the issue that a wrong scope is used when generating view caches for error pages (`UNKNOWN_SCOPE` instead of `viewcache`).

Please see the [corresponding JIRA issue](https://jira.ez.no/browse/EZP-20992) for more detailed information. Thanks to @m-keil for the detailed description and fix.