Use correct translation key for errors (#18342)

- Noted by:
https://github.com/go-gitea/gitea/issues/17876#issuecomment-1017503614

routers/web/org/setting.go

@@ -74,10 +74,14 @@ func SettingsPost(ctx *context.Context) {
 			ctx.RenderWithErr(ctx.Tr("form.username_been_taken"), tplSettingsOptions, &form)
 			return
 		} else if err = user_model.ChangeUserName(org.AsUser(), form.Name); err != nil {
-			if db.IsErrNameReserved(err) || db.IsErrNamePatternNotAllowed(err) {
+			switch {
+			case db.IsErrNameReserved(err):
 				ctx.Data["OrgName"] = true
-				ctx.RenderWithErr(ctx.Tr("form.illegal_username"), tplSettingsOptions, &form)
-			} else {
+				ctx.RenderWithErr(ctx.Tr("repo.form.name_reserved", err.(db.ErrNameReserved).Name), tplSettingsOptions, &form)
+			case db.IsErrNamePatternNotAllowed(err):
+				ctx.Data["OrgName"] = true
+				ctx.RenderWithErr(ctx.Tr("repo.form.name_pattern_not_allowed", err.(db.ErrNamePatternNotAllowed).Pattern), tplSettingsOptions, &form)
+			default:
 				ctx.ServerError("ChangeUserName", err)
 			}
 			return