kernel error 3 (not available) 404 errors are being cached

When a content, for some reason, is not available from the DB repository, the legacy view results in a kernel error 3 - not available (http result status 404)

In certain cases, this can be caused by temporary issues (database/content/etc), but the error will be cached in the content view.

This means that a frontend will cache the error (in legacy view cache), despite the fact that the problem may no longer be occurring.

Given the nature of this error, which is different from - for example - permission checks (access denied), it would be expected that they are not cached.

TODO: Document outcome of EZP-27621 in 5.x and 1.x doc.