Test for undefined Bookreader components (e.g. $BRnav)

**Description**

re: #161

In certain cases where no nav bar is present for a book (e.g. a book w/ admin privileges w/ no preview), we can’t expect a nav bar to appear, causing `br.refs.$BRnav` to be `undefined`.

> What problem are we solving? What does the experience look like today? What are the symptoms? How can one reproduce the problem?

**Evidence / Screenshot (if possible)**

> Include evidence of the problem or provide a screenshot.

The bookreader won't fully load due to js error halting execution. In today's regression case, the "admin" bar failed to load (which allows users to QA a book). This is a mission critical use case for our scanning centers to do QA and so such an admin privileged book should be included in our testing:

https://archive.org/details/alopeciaareata0000levy