Polymorphic schema with sub-schema fails with AttributeError during errored validation

[patrick-ryan]

Here's an example:

In [3]: class TestTarget2(Schema): 
   ...:     test = f.String() 
   ...:  
   ...: class TestTargetPoly(PolySchema): 
   ...:     _type = f.String() 
   ...:     __poly_on__ = _type 
   ...:  
   ...: class TestTarget(TestTargetPoly): 
   ...:     test = f.Schema(TestTarget2) 
   ...:     __poly_id__ = 'test' 
   ...:  
   ...: class TestSource(Schema): 
   ...:     target = f.Schema(TestTargetPoly)                                                                   

In [4]: TestSource().validate({'target': {'_type': 'test', 'test': None}})                                      
---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
~/.virtualenvs/docgen/local/lib/python3.6/site-packages/ciri/fields.py in validate(self, value)
    365         try:
--> 366             return schema.validate(value, exclude=self.exclude, whitelist=self.whitelist, tags=self.tags, **schema._schema._validation_opts)
    367         except ValidationError as e:

~/.virtualenvs/docgen/local/lib/python3.6/site-packages/ciri/core.py in validate(self, data, *args, **kwargs)
    849         schema = self.__poly_mapping__.get(id_)(*self.__poly_args__, **self.__poly_kwargs__)
--> 850         return schema.validate(data, *args, **kwargs)
    851 

~/.virtualenvs/docgen/local/lib/python3.6/site-packages/ciri/core.py in validate(self, data, halt_on_error, key_cache, exclude, whitelist, tags, context)
    631         if self._config.raise_errors and errors:
--> 632             raise ValidationError(self)
    633         return valid

ValidationError: <__main__.TestTarget object at 0x7f59e612feb8>

During handling of the above exception, another exception occurred:

FieldValidationError                      Traceback (most recent call last)
~/.virtualenvs/docgen/local/lib/python3.6/site-packages/ciri/core.py in _iterate(self, fields, elements, data, validation_opts, parent, do_serialize, exclude, do_deserialize, do_validate, whitelist, tags)
    445                     if do_validate:
--> 446                         sub = klass_value = valid[key] = parent._subfields[key].validate(sub)
    447                     if do_serialize:

~/.virtualenvs/docgen/local/lib/python3.6/site-packages/ciri/fields.py in validate(self, value)
    367         except ValidationError as e:
--> 368             raise FieldValidationError(FieldError(self, 'invalid', errors=schema._raw_errors))
    369 

FieldValidationError: <ciri.fields.FieldError object at 0x7f59e612ff60>

During handling of the above exception, another exception occurred:

AttributeError                            Traceback (most recent call last)
<ipython-input-4-a26d5cd6636a> in <module>
----> 1 TestSource().validate({'target': {'_type': 'test', 'test': None}})

~/.virtualenvs/docgen/local/lib/python3.6/site-packages/ciri/core.py in validate(self, data, halt_on_error, key_cache, exclude, whitelist, tags, context)
    622                                       self._validation_opts, parent=self,
    623                                       do_validate=True, exclude=exclude,
--> 624                                       whitelist=whitelist, tags=tags)
    625 
    626         if hasattr(self._schema_callables, 'post_validate'):

~/.virtualenvs/docgen/local/lib/python3.6/site-packages/ciri/core.py in _iterate(self, fields, elements, data, validation_opts, parent, do_serialize, exclude, do_deserialize, do_validate, whitelist, tags)
    450                         sub = klass_value = valid[key] = parent._subfields[key].deserialize(sub)
    451                 except ValidationError as _e:
--> 452                     errors[key] = FieldError(parent._subfields[key], 'invalid', errors=field._raw_errors)
    453                 continue
    454 

AttributeError: 'TestTarget' object has no attribute '_raw_errors'