fix: error.cause from undici may be instance of Error (#643)

Fix how Undici network errors are surfaced

src/fetch-wrapper.ts

@@ -136,12 +136,12 @@ export default function fetchWrapper(
       // undici throws a TypeError for network errors
       // and puts the error message in `error.cause`
       // https://github.com/nodejs/undici/blob/e5c9d703e63cd5ad691b8ce26e3f9a81c598f2e3/lib/fetch/index.js#L227
-      if (
-        error instanceof TypeError &&
-        "cause" in error &&
-        typeof error.cause === "string"
-      ) {
-        message = error.cause;
+      if (error.name === "TypeError" && "cause" in error) {
+        if (error.cause instanceof Error) {
+          message = error.cause.message;
+        } else if (typeof error.cause === "string") {
+          message = error.cause;
+        }
       }
 
       throw new RequestError(message, 500, {

test/request.test.ts

@@ -439,7 +439,29 @@ x//0u+zd/R/QRUzLOw4N72/Hu+UG6MNt5iDZFCtapRaKt6OvSBwy8w==
       });
   });
 
-  it("Request TypeError error", () => {
+  it("Request TypeError error with an Error cause", () => {
+    const mock = fetchMock.sandbox().get("https://127.0.0.1:8/", {
+      throws: Object.assign(new TypeError("fetch failed"), {
+        cause: new Error("bad"),
+      }),
+    });
+
+    // port: 8 // officially unassigned port. See https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers
+    return request("GET https://127.0.0.1:8/", {
+      request: {
+        fetch: mock,
+      },
+    })
+      .then(() => {
+        throw new Error("should not resolve");
+      })
+      .catch((error) => {
+        expect(error.status).toEqual(500);
+        expect(error.message).toEqual("bad");
+      });
+  });
+
+  it("Request TypeError error with a string cause", () => {
     const mock = fetchMock.sandbox().get("https://127.0.0.1:8/", {
       throws: Object.assign(new TypeError("fetch failed"), { cause: "bad" }),
     });