fix(migration): return proper error message when request fails

Related to https://github.com/go-vikunja/vikunja/issues/1788

pkg/modules/migration/helpers.go

@@ -20,6 +20,8 @@ import (
 	"bytes"
 	"context"
 	"crypto/rand"
+	"fmt"
+	"io"
 	"math"
 	"math/big"
 	"net/http"
@@ -92,9 +94,19 @@ func DoPostWithHeaders(url string, form url.Values, headers map[string]string) (
 			return resp, nil
 		}
 
-		// Don't retry on last attempt
+		// Return error on last attempt if still getting 5xx
 		if attempt == maxRetries-1 {
-			return resp, nil
+			bodyBytes, readErr := io.ReadAll(resp.Body)
+			resp.Body.Close()
+
+			// Re-create the body so the caller can still read it if needed
+			resp.Body = io.NopCloser(bytes.NewBuffer(bodyBytes))
+
+			if readErr != nil {
+				return resp, fmt.Errorf("request failed after %d attempts with status code %d (could not read response body: %w)", maxRetries, resp.StatusCode, readErr)
+			}
+
+			return resp, fmt.Errorf("request failed after %d attempts with status code %d: %s", maxRetries, resp.StatusCode, string(bodyBytes))
 		}
 
 		// Close the body before retrying
@@ -108,5 +120,5 @@ func DoPostWithHeaders(url string, form url.Values, headers map[string]string) (
 		time.Sleep(delay + jitter)
 	}
 
-	return resp, nil
+	return nil, fmt.Errorf("request failed after %d attempts", maxRetries)
 }

pkg/modules/migration/helpers_test.go

@@ -20,6 +20,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strings"
 	"sync/atomic"
 	"testing"
 )
@@ -53,17 +54,25 @@ func TestDoPostWithHeaders_RetriesOn500(t *testing.T) {
 
 func TestDoPostWithHeaders_GivesUpAfter3Retries(t *testing.T) {
 	var attempts atomic.Int32
+	expectedBody := "Internal Server Error: database connection failed"
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
 		attempts.Add(1)
 		w.WriteHeader(http.StatusInternalServerError)
+		_, _ = w.Write([]byte(expectedBody))
 	}))
 	defer server.Close()
 
 	form := url.Values{"key": {"value"}}
 	resp, err := DoPostWithHeaders(server.URL, form, map[string]string{})
 
-	if err != nil {
-		t.Fatalf("expected no error, got %v", err)
+	if err == nil {
+		t.Fatal("expected error after exhausted retries, got nil")
+	}
+	if !strings.Contains(err.Error(), expectedBody) {
+		t.Errorf("expected error message to contain response body %q, got: %s", expectedBody, err.Error())
+	}
+	if resp == nil {
+		t.Fatal("expected response to be returned with error, got nil")
 	}
 	defer resp.Body.Close()
 	if resp.StatusCode != http.StatusInternalServerError {