return string from onError in downstream type signatures

Author: salazarm

packages/react-dom/src/server/ReactDOMFizzServerBrowser.js

@@ -32,7 +32,7 @@ type Options = {|
   bootstrapModules?: Array<string>,
   progressiveChunkSize?: number,
   signal?: AbortSignal,
-  onError?: (error: mixed) => void,
+  onError?: (error: mixed) => ?string,
 |};
 
 // TODO: Move to sub-classing ReadableStream.

packages/react-noop-renderer/src/ReactNoopServer.js

@@ -253,7 +253,7 @@ type Options = {
   progressiveChunkSize?: number,
   onShellReady?: () => void,
   onAllReady?: () => void,
-  onError?: (error: mixed) => void,
+  onError?: (error: mixed) => ?string,
 };
 
 function render(children: React$Element<any>, options?: Options): Destination {