title: Marshmallow not giving errors
body: <p>If I used Marshmallow to create a schema like this:</p>
  
  <pre><code>class TempSchema(Schema):
      id = fields.Int()
      email = fields.Str(required=True,
                     validate=validate.Email(error='Not a valid email address'))
      password = fields.Str(required=True,
                        validate=[validate.Length(min=6, max=36)],
                        load_only=True)
  </code></pre>
  
  <p>and then I do something like:</p>
  
  <pre><code>temp = TempSchema()
  temp.dumps({'email':123})
  </code></pre>
  
  <p>I'd expect an error but I get:</p>
  
  <pre><code>MarshalResult(data='{"email": "123"}', errors={})
  </code></pre>
  
  <p>Why isn't this or anything else showing up as an error?</p>