title: How to make marshmallow raise AttributeErrors on serialization
body: <p>How can I make marshmallow make raise an exception when data is missing attributes? </p>
  
  <p>Consider the following example:</p>
  
  <pre><code>In [8]: import marshmallow
  
  In [9]: class Foo(marshmallow.Schema):
     ...:     bar = marshmallow.fields.Str(required=True)
     ...:
  
  In [10]: class Bar:
      ...:     pass
      ...:
  
  In [11]: bar = Bar()
  
  In [12]: Foo().dumps(bar)
  Out[12]: MarshalResult(data='{}', errors={})
  
  In [13]: bar.bar
  ---------------------------------------------------------------------------
  AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-13-6901e83d9f0c&gt; in &lt;module&gt;()
  ----&gt; 1 bar.bar
  
  AttributeError: 'Bar' object has no attribute 'bar'
  </code></pre>
  
  <p>What's even worse, is this:</p>
  
  <pre><code>In [14]: foo.loads(foo.dumps(bar).data)
  Out[14]: UnmarshalResult(data={}, errors={'bar': ['Missing data for required field.']})
  </code></pre>