title: Is it possible to add errors to an ActiveRecord object without associating them with a particular attribute?
body: <p>If you need to code a considerably complex validation, the error sometimes doesnt lie in a particular attribute, but in a combination of several of them.</p>
  
  <p>For example, if i want to validate that a the time period between :start_date and :end_date doesnt contain any sunday, the error doesnt belong specifically to either of those fields, but  the Errors add method requires to specify it. </p>