How to use form_set_error if no form field is involved in the error?
There is a plain solution

form_set_error('form',$error_messages);


Does not highlight any field, and the validation fails as expected. It doesn't allow multiple message, but this can be solved using a little hack

$error_messages = "";
if (type_error_1)
   $error_messages .= "Error Message on First Line &lt;br/&gt;";
}
if (type_error_2){
   $error_messages .= "Error Message on Another Line &lt;br/&gt;";
}
if(strlen($error_messages) &gt; 0){
  form_set_error('form',$error_messages);
}