Bug 1889417 - On errors, reject the document.l10n.ready promise in tests. r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D206587

dom/l10n/DocumentL10n.cpp

@@ -93,8 +93,15 @@ class L10nReadyHandler final : public PromiseNativeHandler {
      * rejection" warning, which is noisy and not-actionable.
      *
      * So instead, we just resolve and report errors.
+     *
+     * However, in automated tests we do reject so that errors with missing
+     * messages and resources can be caught.
      */
-    mPromise->MaybeResolveWithUndefined();
+    if (xpc::IsInAutomation()) {
+      mPromise->MaybeRejectWithClone(aCx, aValue);
+    } else {
+      mPromise->MaybeResolveWithUndefined();
+    }
   }
 
  private:

dom/l10n/tests/mochitest/document_l10n/test_docl10n_ready_rejected.html

@@ -12,12 +12,17 @@
 
   document.addEventListener("DOMContentLoaded", async function() {
       /**
-       * Even when we fail to localize all elements, we will
-       * still resolve the `ready` promise to communicate that
+       * Outside of tests, even when we fail to localize all elements,
+       * we will still resolve the `ready` promise to communicate that
        * the initial translation phase is now completed.
+       *
+       * In tests, the promise will be rejected to allow errors to be caught.
        */
       document.l10n.ready.then(() => {
-        is(1, 1, "the ready should resolve");
+        is(1, 2, "the ready should not resolve");
+        SimpleTest.finish();
+      }, (_err) => {
+        is(1, 1, "the ready should reject");
         SimpleTest.finish();
       });
   });