[Browser] Return a loading page from the service worker if the web worker isn't ready yet

[adamziel]

What problem is this issue looking to solve?

The service worker passes requests to .php files to the web worker. However, the web worker may not be loaded yet or it could be busy at the moment.

How does this issue proposes to solve it?

Reply with HTTP 502 or HTTP 408 eventually somewhere here:

https://github.com/adamziel/wordpress-wasm/blob/9d04a113a97f53a1303fefba642f65f1b5a5a5f6/src/web/service-worker.js#L31-L52

[adamziel]

This shouldn't be an issue anymore as the initialization pipeline should prevent this situation from arising in the first place.
Returning a loading page is tricky anyway as it not always clear what kind of response is expected. If this will cause any problems in the future, let's first look into intercepting requests in the service worker only after the WASM WordPress is up and running.