Error if we can't complete the root synchronously

Maybe this should always error but in the async forms we can just delay
the stream until it resolves so it does have some useful semantics.

In the synchronous form it's never useful though. I'm mostly adding the
error because we're testing this behavior for renderToString specifically.

packages/react-dom/src/server/ReactDOMLegacyServerBrowser.js

@@ -51,6 +51,11 @@ function renderToString(
       fatalError = error;
     },
   };
+
+  let readyToStream = false;
+  function onReadyToStream() {
+    readyToStream = true;
+  }
   const request = createRequest(
     children,
     destination,
@@ -59,7 +64,7 @@ function renderToString(
     Infinity,
     onError,
     undefined,
-    undefined,
+    onReadyToStream,
   );
   startWork(request);
   // If anything suspended and is still pending, we'll abort it before writing.
@@ -69,6 +74,13 @@ function renderToString(
   if (didFatal) {
     throw fatalError;
   }
+  invariant(
+    readyToStream,
+    'A React component suspended while rendering, but no fallback UI was specified.\n' +
+      '\n' +
+      'Add a <Suspense fallback=...> component higher in the tree to ' +
+      'provide a loading indicator or placeholder to display.',
+  );
   return result;
 }