Fix silently ignored error when saving outbound test URL setting (#4209)

In the Xray settings update handler, the error from
SetXrayOutboundTestUrl was silently discarded. If the database write
failed, the user received a success toast ("Settings updated
successfully") but the outbound test URL was not actually saved.

Now properly checks the error and returns a failure response to the
user, consistent with how the preceding SaveXraySetting call is
handled.

web/controller/xray_setting.go

@@ -104,7 +104,10 @@ func (a *XraySettingController) updateSetting(c *gin.Context) {
 	if outboundTestUrl == "" {
 		outboundTestUrl = "https://www.google.com/generate_204"
 	}
-	_ = a.SettingService.SetXrayOutboundTestUrl(outboundTestUrl)
+	if err := a.SettingService.SetXrayOutboundTestUrl(outboundTestUrl); err != nil {
+		jsonMsg(c, I18nWeb(c, "pages.settings.toasts.modifySettings"), err)
+		return
+	}
 	jsonMsg(c, I18nWeb(c, "pages.settings.toasts.modifySettings"), nil)
 }