FIX: Bookmark list showing wrong no content message (PR #10501)

Meta report: https://meta.discourse.org/t/spurious-permissions-error-after-deleting-last-bookmark/161289

When deleting all bookmarks in their list, users were seeing an “access denied” message instead of the regular no content message. This is because when we were calling loadMore and no further results were returned, we were presuming the null response from the resolved promise meant that there was a permissions error. Fixed this and moved the message into a computed property to show the correct message.

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https://meta.discourse.org/t/spurious-permissions-error-after-deleting-last-bookmark/161289/5