Make error screens more visually consistent (#2951)

src/RichError.tsx

@@ -0,0 +1,48 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only
+Please see LICENSE in the repository root for full details.
+*/
+
+import { type FC, type ReactNode } from "react";
+import { useTranslation } from "react-i18next";
+import { PopOutIcon } from "@vector-im/compound-design-tokens/assets/web/icons";
+
+import { ErrorView } from "./ErrorView";
+
+/**
+ * An error consisting of a terse message to be logged to the console and a
+ * richer message to be shown to the user, as a full-screen page.
+ */
+export class RichError extends Error {
+  public constructor(
+    message: string,
+    /**
+     * The pretty, more helpful message to be shown on the error screen.
+     */
+    public readonly richMessage: ReactNode,
+  ) {
+    super(message);
+  }
+}
+
+const OpenElsewhere: FC = () => {
+  const { t } = useTranslation();
+
+  return (
+    <ErrorView Icon={PopOutIcon} title={t("error.open_elsewhere")}>
+      <p>
+        {t("error.open_elsewhere_description", {
+          brand: import.meta.env.VITE_PRODUCT_NAME || "Element Call",
+        })}
+      </p>
+    </ErrorView>
+  );
+};
+
+export class OpenElsewhereError extends RichError {
+  public constructor() {
+    super("App opened in another tab", <OpenElsewhere />);
+  }
+}