Tabs with missing titles are hard to diagnose

[wilkinsona]

When a tab as no title, the Javascript fails with an error like the following:

TypeError: null is not an object (evaluating 't.querySelector(".title").textContent')

It would be good to catch these missing titles at build time if we can. Failing that, it would be good to report a better error so that it's easier to identify the places where the titles are missing.

/cc @artembilan @Buzzardo