When an error is encountered, this variable will be initialized with the message of the error and it's will be available from the /error/ page

<#error_message>
<#if condition="'<#error_btn_back_url>' <> ''">
  <br />
  <#if condition="'<#error_btn_back_method>' = 'POST'">
    <form action="<#error_btn_back_url>" method="post" name="ak_back_form" class="form1">
      <#for var="i" from="0" to="<#error_btn_back_post_params.count> - 1">
        <input type="hidden" name="<#error_btn_back_post_params.Names[<#i>]>" value="<#error_btn_back_post_params.ValueFromIndex[<#i>]>" />
      <#endfor>
    </form>
    <#htmlbutton template="button_4_1_2.html"
                 href="javascript:void(0);"
                 onclick="document.forms['ak_back_form'].submit(); return false;"
                 caption="Previous">
  <#else>
    <#htmlbutton template="button_4_1_2.html"
                 href="<#error_btn_back_url>"
                 caption="Previous">
  <#endif>  
<#endif>