Minify "no fallback UI specified" error in prod

This error message wasn't being minified because it doesn't use
invariant. The reason it didn't use invariant is because this particular
error is created without begin thrown — it doesn't need to be thrown
because it's located inside the error handling part of the runtime.

Now that the error minification script supports Error constructors, we
can minify it by assigning it a production error code in
`scripts/error-codes/codes.json`.

To support the use of Error constructors more generally, I will add a
lint rule that enforces each message has a corresponding error code.

packages/react-reconciler/src/ReactFiberThrow.new.js

@@ -474,7 +474,8 @@ function throwException(
       return;
     } else {
       // No boundary was found. Fallthrough to error mode.
-      // TODO: Use invariant so the message is stripped in prod?
+      // TODO: We should never call getComponentNameFromFiber in production.
+      // Log a warning or something to prevent us from accidentally bundling it.
       value = new Error(
         (getComponentNameFromFiber(sourceFiber) || 'A React component') +
           ' suspended while rendering, but no fallback UI was specified.\n' +

packages/react-reconciler/src/ReactFiberThrow.old.js

@@ -474,7 +474,8 @@ function throwException(
       return;
     } else {
       // No boundary was found. Fallthrough to error mode.
-      // TODO: Use invariant so the message is stripped in prod?
+      // TODO: We should never call getComponentNameFromFiber in production.
+      // Log a warning or something to prevent us from accidentally bundling it.
       value = new Error(
         (getComponentNameFromFiber(sourceFiber) || 'A React component') +
           ' suspended while rendering, but no fallback UI was specified.\n' +

scripts/error-codes/codes.json

@@ -394,5 +394,6 @@
   "405": "hydrateRoot(...): Target container is not a DOM element.",
   "406": "act(...) is not supported in production builds of React.",
   "407": "Missing getServerSnapshot, which is required for server-rendered content. Will revert to client rendering.",
-  "408": "Missing getServerSnapshot, which is required for server-rendered content."
+  "408": "Missing getServerSnapshot, which is required for server-rendered content.",
+  "409": "%s suspended while rendering, but no fallback UI was specified.\n\nAdd a <Suspense fallback=...> component higher in the tree to provide a loading indicator or placeholder to display."
 }