Fix the 'puzzle doesn't work' apology, which I'd accidentally moved

Fix the 'puzzle doesn't work' apology, which I'd accidentally moved
inside the top-level display:none puzzle container as a side effect of
r9809.

Also, while I'm at it, reword the apology to mention typed arrays as
the most likely cause of failure (AFAIK that's the most modern feature
required by the JS front end), and fix indecision between singular and
plural ('this puzzle' doesn't work, perhaps a feature 'they depend on'
is missing).

[originally from svn r9818]
[r9809 == 5dc559c8be1b8f6ed15f560433f25c952c874f93]