MM-37576: Fix reference to incorrect error variable (#18066)

We use the correct error variable returned from
the store method prior to it.

https://mattermost.atlassian.net/browse/MM-37576

```release-note
NONE
```

app/user.go

@@ -2275,7 +2275,7 @@ func (a *App) UpdateThreadReadForUser(userID, teamID, threadID string, timestamp
 	}
 	membership, storeErr := a.Srv().Store.Thread().MaintainMembership(userID, threadID, opts)
 	if storeErr != nil {
-		return nil, model.NewAppError("UpdateThreadReadForUser", "app.user.update_thread_read_for_user.app_error", nil, err.Error(), http.StatusInternalServerError)
+		return nil, model.NewAppError("UpdateThreadReadForUser", "app.user.update_thread_read_for_user.app_error", nil, storeErr.Error(), http.StatusInternalServerError)
 	}
 
 	post, err := a.GetSinglePost(threadID)

app/user_test.go

@@ -22,7 +22,9 @@ import (
 	"github.com/mattermost/mattermost-server/v6/einterfaces/mocks"
 	"github.com/mattermost/mattermost-server/v6/model"
 	oauthgitlab "github.com/mattermost/mattermost-server/v6/model/gitlab"
+	"github.com/mattermost/mattermost-server/v6/services/users"
 	"github.com/mattermost/mattermost-server/v6/store"
+	storemocks "github.com/mattermost/mattermost-server/v6/store/storetest/mocks"
 	"github.com/mattermost/mattermost-server/v6/utils/testutils"
 )
 
@@ -1508,14 +1510,15 @@ func TestPatchUser(t *testing.T) {
 func TestUpdateThreadReadForUser(t *testing.T) {
 	os.Setenv("MM_FEATUREFLAGS_COLLAPSEDTHREADS", "true")
 	defer os.Unsetenv("MM_FEATUREFLAGS_COLLAPSEDTHREADS")
-	th := Setup(t).InitBasic()
-	defer th.TearDown()
-	th.App.UpdateConfig(func(cfg *model.Config) {
-		*cfg.ServiceSettings.ThreadAutoFollow = true
-		*cfg.ServiceSettings.CollapsedThreads = model.CollapsedThreadsDefaultOn
-	})
 
 	t.Run("Ensure thread membership is created and followed", func(t *testing.T) {
+		th := Setup(t).InitBasic()
+		defer th.TearDown()
+		th.App.UpdateConfig(func(cfg *model.Config) {
+			*cfg.ServiceSettings.ThreadAutoFollow = true
+			*cfg.ServiceSettings.CollapsedThreads = model.CollapsedThreadsDefaultOn
+		})
+
 		rootPost, appErr := th.App.CreatePost(th.Context, &model.Post{UserId: th.BasicUser2.Id, CreateAt: model.GetMillis(), ChannelId: th.BasicChannel.Id, Message: "hi"}, th.BasicChannel, false, false)
 		require.Nil(t, appErr)
 		replyPost, appErr := th.App.CreatePost(th.Context, &model.Post{RootId: rootPost.Id, UserId: th.BasicUser2.Id, CreateAt: model.GetMillis(), ChannelId: th.BasicChannel.Id, Message: "hi"}, th.BasicChannel, false, false)
@@ -1536,6 +1539,34 @@ func TestUpdateThreadReadForUser(t *testing.T) {
 		require.NotNil(t, threadMembership)
 		assert.True(t, threadMembership.Following)
 	})
+
+	t.Run("Ensure no panic on error", func(t *testing.T) {
+		th := SetupWithStoreMock(t)
+		defer th.TearDown()
+
+		mockStore := th.App.Srv().Store.(*storemocks.Store)
+		mockUserStore := storemocks.UserStore{}
+		mockUserStore.On("Count", mock.Anything).Return(int64(10), nil)
+		mockUserStore.On("Get", mock.Anything, "user1").Return(&model.User{Id: "user1"}, nil)
+
+		mockThreadStore := storemocks.ThreadStore{}
+		mockThreadStore.On("MaintainMembership", "user1", "postid", mock.Anything).Return(nil, errors.New("error"))
+
+		var err error
+		th.App.srv.userService, err = users.New(users.ServiceConfig{
+			UserStore:    &mockUserStore,
+			SessionStore: &storemocks.SessionStore{},
+			OAuthStore:   &storemocks.OAuthStore{},
+			ConfigFn:     th.App.srv.Config,
+			LicenseFn:    th.App.srv.License,
+		})
+		require.NoError(t, err)
+		mockStore.On("User").Return(&mockUserStore)
+		mockStore.On("Thread").Return(&mockThreadStore)
+
+		_, err = th.App.UpdateThreadReadForUser("user1", "team1", "postid", 100)
+		require.Error(t, err)
+	})
 }
 
 func TestCreateUserWithInitialPreferences(t *testing.T) {