Another rewrite of the WASM apology message.

Another rewrite of the WASM apology message.

Now I've had a chance to collect some more data from browser users,
it's got a list of browsers in which we've definitely seen the WASM
puzzle working (so that if _your_ instance of one of those browsers
fails, you should check it for problems at your end, like
configuration details or overzealous web filtering software), and some
thoughts about what to report.

The result is a lot longer than the previous error message, so I've
put the bulk of it in a <details>. That way it won't look so silly
when it initially flashes up while things are loading.