Add `app/` directory support for custom title in Error page

### Describe the feature you'd like to request I would like to have the possibility to control the page title, description etc. in the error and not-found page. currently Its possible to get around not-found page by creating a conditional in metadata function for the not-found page but is still not nice. ### Describe the solution you'd like I would like not-found.js and error.js accept `metadata` and `generateMetadata()` similar to `page.js` and `layout.js` ```jsx // app/entity/[entityId]/not-found.js export async function generateMetadata(props) { return { title: `entity ${props.params.entityId} not found`, description: `There is no entity with id: ${props.params.entityId}` } } export default function EntityNotFound() { return <div>Sorry this Entity was not found</div>; } ``` ```jsx // app/error.js export const metadata = { title: "Error", description: "ups something went wrong" } export default function Error() { return <div>Sorry this Entity was not found</div>; } ``` ### Describe alternatives you've considered for not-found, maybe allow to pass the metadata object as an argument for the function ```jsx export default async function AnimalPage(props) { const singleEntity = await getAnimalById(props.params.entityId); if (!singleEntity) { const metadata = { title: `entity ${props.params.entityId} not found`, description: `There is no entity with id: ${props.params.entityId}` } notFound(metadata); } return <> ... rest of the component } ```