title: Document "xyz" could not be saved. Multiple validation errors occured
body: <p>Apologies if this is a bit vague, but that's half of the problem.</p>
  
  <p>I have a document based core data app that I am working on, it is doing what it's told while running and doesn't generate any errors. But when the user saves the document, the document pops up a 'The document "xyz" could not be saved as "xyz". Multiple validation errors occurred' alert.</p>
  
  <p>My question is - where do you start looking to fix / debug this? As the program does not fall over in the debugger I have no stack trace etc. Is this liable to be a faulty Entity relationship, or no data being saved in a non-optional Attribute of an Entity or... Is there a way to tell exactly what is failing validation?</p>
  
  <p>Any suggestions of the best way to proceed greatly appreciated.</p>
  
  <p>Related to this, what / how is the best method to catch such an error in the future so it doesn't get as far as the user. </p>
  
  <p>Many Thanks</p>