Based on Solspace Freeform documnetation
The Error Page function allows you to bypass the native EE error message template and create and customize your own. Simply create a new EE template and add in the applicable variables below, along with anything else you typically use in your templates (template tags, variables, or embeds). This function is available for all use in all form functions such as Freeform:Form, Freeform:Composer, etc. Make sure you specify the error_page="freeform/error_template" parameter in the form you'd like to use this error template.
but where I can define
error_page="freeform/error_template" parameter when I'm on a composer editor? I have try to add it to the Composer Template as Template Params but still is using default EE errors message page.