When attempting to subscribe other user to issue report why access denied (#18091)

Fix #18090

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/api/v1/repo/issue_subscription.go

@@ -5,6 +5,7 @@
 package repo
 
 import (
+	"fmt"
 	"net/http"
 
 	"code.gitea.io/gitea/models"
@@ -128,7 +129,7 @@ func setIssueSubscription(ctx *context.APIContext, watch bool) {
 
 	//only admin and user for itself can change subscription
 	if user.ID != ctx.User.ID && !ctx.User.IsAdmin {
-		ctx.Error(http.StatusForbidden, "User", nil)
+		ctx.Error(http.StatusForbidden, "User", fmt.Errorf("%s is not permitted to change subscriptions for %s", ctx.User.Name, user.Name))
 		return
 	}