Return 404 when thread membership not found (#21960)

We were incorrectly returning by checking
for `err != nil` when a 404 error was included
in that condition.

```release-note
NONE
```

app/user.go

@@ -2484,24 +2484,31 @@ func (a *App) GetThreadsForUser(userID, teamID string, options model.GetUserThre
 }
 
 func (a *App) GetThreadMembershipForUser(userId, threadId string) (*model.ThreadMembership, *model.AppError) {
-	threadMembership, err := a.Srv().Store().Thread().GetMembershipForUser(userId, threadId)
-	if err != nil {
-		return nil, model.NewAppError("GetThreadMembershipForUser", "app.user.get_thread_membership_for_user.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
-	}
-	if threadMembership == nil {
-		return nil, model.NewAppError("GetThreadMembershipForUser", "app.user.get_thread_membership_for_user.not_found", nil, "thread membership not found/followed", http.StatusNotFound)
+	threadMembership, nErr := a.Srv().Store().Thread().GetMembershipForUser(userId, threadId)
+	if nErr != nil {
+		var nfErr *store.ErrNotFound
+		switch {
+		case errors.As(nErr, &nfErr):
+			return nil, model.NewAppError("GetThreadMembershipForUser", "app.user.get_thread_membership_for_user.not_found", nil, "", http.StatusNotFound).Wrap(nErr)
+		default:
+			return nil, model.NewAppError("GetThreadMembershipForUser", "app.user.get_thread_membership_for_user.app_error", nil, "", http.StatusInternalServerError).Wrap(nErr)
+		}
 	}
 	return threadMembership, nil
 }
 
 func (a *App) GetThreadForUser(threadMembership *model.ThreadMembership, extended bool) (*model.ThreadResponse, *model.AppError) {
-	thread, err := a.Srv().Store().Thread().GetThreadForUser(threadMembership, extended, a.isPostPriorityEnabled())
-	if err != nil {
-		return nil, model.NewAppError("GetThreadForUser", "app.user.get_threads_for_user.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
-	}
-	if thread == nil {
-		return nil, model.NewAppError("GetThreadForUser", "app.user.get_threads_for_user.not_found", nil, "thread not found/followed", http.StatusNotFound)
+	thread, nErr := a.Srv().Store().Thread().GetThreadForUser(threadMembership, extended, a.isPostPriorityEnabled())
+	if nErr != nil {
+		var nfErr *store.ErrNotFound
+		switch {
+		case errors.As(nErr, &nfErr):
+			return nil, model.NewAppError("GetThreadForUser", "app.user.get_threads_for_user.not_found", nil, "thread not found/followed", http.StatusNotFound)
+		default:
+			return nil, model.NewAppError("GetThreadForUser", "app.user.get_threads_for_user.app_error", nil, "", http.StatusInternalServerError).Wrap(nErr)
+		}
 	}
+
 	a.sanitizeProfiles(thread.Participants, false)
 	thread.Post.SanitizeProps()
 	return thread, nil