Description
This method may also be called by a custom error message generator 
      to fill in template values from the params array, simply  set the third
      parameter to the error message template string to use
     
      The special variable %__msg% is reserved: use it only to specify  where a message passed
      in by the user should be placed in the template,  like so:
     
Error message: %msg% - internal error
If the message passed like so:
      
| <?php
$stack->push(ERROR_CODE, 'error', array(), 'server error 500');
?> | 
     The returned error message will be "Error message: server error 500 -  internal error"