Confusing error message when following Bouncer links to content behind a login

Surfaced by @dwolfe while he was looking at https://hypothes.is/search.

## Steps to reproduce
1. From activity pages or from https://hypothes.is/search, find an annotation made on a page or document which is hidden behind a paywall/login. For example, these annotations made in the Public channel on a PDF which is hosted behind a login (link provided below):

2. Click on the "Visit annotations in context" link. (In this case: https://hyp.is/xDikIrXEEeiDWyedk5LgNQ/estudijas.lu.lv/pluginfile.php/625538/mod_resource/content/0/Crewe%2C%20E.%20%20Harrison%2C%20E.%201999.%20Seeing%20Culture%20as%20a%20Barrier.pdf)

3. You'll be redirected to https://estudijas.lu.lv/auth/shibboleth/login.php#annotations:xDikIrXEEeiDWyedk5LgNQ and the sidebar shows an error message: "You do not have permission to view this annotation."

The error message "You do not have permission to view this annotation." is confusing, because the annotation itself is visible from the Hypothesis search page, and it's in the Public channel. It's the _document_ which we don't have permission to view.