Show a reasonable error in the HTML when the Shinylive JS Service Worker fails to load

[georgestagg]

In at least some situations the above simply causes a white blank screen. We should detect and throw a visible error when:

• Shinylive is not running under a web server, and so...
• The Service Worker fails to load.

With some reasonable text to explain what has happened.

[gadenbuie]

Another scenario to try to detect: when the service worker fails to load because recent updates to shinylive require a reload. In these cases, it's worth giving any kind of hint that reloading the page (or clearing the cache and reloading) is worth trying; these are otherwise quite confusing error messages. (Especially on https://shinylive.io.)