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)