title: Mongoose unique validation error type
body: <p>I'm using this schema with <code>mongoose 3.0.3</code> from npm:</p>
  
  <pre><code>var schema = new Schema({
  
      _id: Schema.ObjectId,
      email: {type: String, required: true, unique: true}
  
  });
  </code></pre>
  
  <p>If I try to save a email that is already in db, I expect to get a <code>ValidationError</code> like if a <code>required</code> field is omitted. However this is not the case, I get a <code>MongoError: E11000 duplicate key error index</code>.</p>
  
  <p>Which is not a validation error (happens even if I remove the unique:true).</p>
  
  <p>Any idea why?</p>