feat: error page on invalid course key (#761)

src/generic/NotFoundAlert.jsx

@@ -0,0 +1,14 @@
+import React from 'react';
+import { FormattedMessage } from '@edx/frontend-platform/i18n';
+import { Alert } from '@edx/paragon';
+
+const NotFoundAlert = () => (
+  <Alert variant="danger" data-testid="notFoundAlert">
+    <FormattedMessage
+      id="authoring.alert.error.notfound"
+      defaultMessage="Not found."
+    />
+  </Alert>
+);
+
+export default NotFoundAlert;