How to use form_set_error if no form field is involved in the error? In a form_validate hook, I have some custom validation and I'm using

form_set_error('field_foo', "error_message");


to show validation errors. If the validation returns an error that is not linked to a particular form field, how do I show the error, indipendently? (For example: the error is you compiled more that three times the form.)

EDIT: I though to use:

drupal_set_message("error_message", 'error');


From the documentaion for 7.x


  When a validation error is detected, the validator calls
  form_set_error() to indicate which element needs to be changed and
  provide an error message.


So, form_set_error() produce an error linked to a particular field of the form. I needed to display error indipendently in a custom complex validation, being errors not really about a field. I though to use form_set_error() on a random field, but it is not the perfect solution, for the field was highlighted and only the last error produced this way was visible.
drupal_set_message solves this two problem, but the validation success and the form continues like there was no 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);
}