Add a modernizr check for WebAssembly support

We don't work at all without this now and currently fail in terrible
ways. This will cause us to display the 'unsupported browser' screen
if we don't have wasm.

Also comment the three different types of error page.

Playwright test coming for this in react-sdk.

src/vector/index.ts

@@ -79,6 +79,10 @@ function checkBrowserFeatures(): boolean {
     // and older Firefox has the former but not the latter, so we add our own.
     window.Modernizr.addTest("intlsegmenter", () => typeof window.Intl?.Segmenter === "function");
 
+    // Basic test for WebAssembly support. We could also try instantiating a simple module,
+    // although this would start to make (more) assumptions about how rust-crypto loads its wasm.
+    window.Modernizr.addTest("wasm", () => typeof WebAssembly === "object" && typeof WebAssembly.Module === "function");
+
     const featureList = Object.keys(window.Modernizr) as Array<keyof ModernizrStatic>;
 
     let featureComplete = true;
@@ -240,6 +244,10 @@ async function start(): Promise<void> {
 }
 
 start().catch((err) => {
+    // If we get here, things have gone terribly wrong and we cannot load the app javascript at all.
+    // Show a different, very simple iframed-static error page. Or actually, one of two different ones
+    // depending on whether the browser is supported (ie. we think we should be able to load but
+    // failed) or unsupported (where we tried anyway and, lo and behold, we failed).
     logger.error(err);
     // show the static error in an iframe to not lose any context / console data
     // with some basic styling to make the iframe full page