Update the serverFunctions.onError reference (#1610)

* Update the `serverFunctions.onError` reference

* Document the failure semantics of the awaited `onError` handler

* Tighten the wording for a failing `onError` handler

Author: adipascu

src/routes/solid-start/v2/reference/config/solid-start.mdx

@@ -139,11 +139,13 @@ solidStart({
 
 - **Type:** `string`
 
-Path to a module whose default export handles values thrown by server functions before SolidStart serializes them into a response.
+Path to a module whose default export handles values thrown by server functions before SolidStart serializes them into a response. Only calls arriving over the network reach it. The module is bundled only into the server, so it can import server-only code.
 
-The handler can report the original value and provide a safer replacement for the client. SolidStart awaits the handler before serializing the response, allowing a monitoring service to flush first.
+The handler can report the original value and provide a safer replacement for the client. It may be `async`: SolidStart awaits it before serializing the response, so a monitoring service can flush first. If the handler itself throws or rejects, the original value is sent as if no handler were configured.
 
-Returning or resolving to `undefined` or `null` preserves the original value. A `Response` preserves control flow, while any other value replaces the original. The module is bundled only into the server, so it can import server-only code.
+- Return `undefined` or `null` to preserve the original value.
+- Return a `Response` unchanged to pass it through, which keeps a thrown `redirect` working. See [Return Responses](/solid-start/v2/advanced/return-responses).
+- Return any other value to send it in place of the original, serialized to the client along with its own properties.
 
 ```tsx title="vite.config.ts"
 solidStart({
@@ -153,17 +155,21 @@ solidStart({
 });
 ```
 
+The module it names, `src/server-function-error.ts`:
+
 ```ts title="src/server-function-error.ts"
 import type { ServerFunctionErrorHandler } from "@solidjs/start/server";
+import { captureException } from "./your-monitoring-client";
 
 const onServerFunctionError: ServerFunctionErrorHandler = (thrown) => {
-	console.error(thrown);
-
-	if (thrown instanceof Error) {
-		return new Error("The server function failed.");
+	// redirect() throws a Response, so returning it preserves that control flow.
+	if (thrown instanceof Response) {
+		return thrown;
 	}
 
-	return undefined;
+	captureException(thrown); // or console.error, or any reporter
+
+	return new Error("The server function failed.");
 };
 
 export default onServerFunctionError;