Field-level validation errors cannot be saved on a nested field with many=True

[nelfin]

When validating a field, all of its error messages are saved and appended to the list of messages for that field name in call_and_store, but for fields.Nested with many=True the error messages for child elements of the collection are saved and stored in a dictionary keyed by the element index. This causes the following exception on this minimal test case:

class Inner(Schema):
    req = fields.Field(required=True)

class Outer(Schema):
    inner = fields.Nested(Inner, many=True)

    @validates('inner')
    def validates_inner(self, data):
        raise ValidationError('not a chance')

outer = Outer()
_, errors = outer.load({'inner': [{}]})

Traceback (most recent call last):
  File "/home/andrew/misc/marshmallow/tests/test_schema.py", line 1238, in test_all_errors_on_many_nested_field_with_validates_decorator
    _, errors = outer.load({'inner': [{}]})
  File "/home/andrew/misc/marshmallow/marshmallow/schema.py", line 539, in load
    result, errors = self._do_load(data, many, partial=partial, postprocess=True)
  File "/home/andrew/misc/marshmallow/marshmallow/schema.py", line 610, in _do_load
    self._invoke_field_validators(data=result, many=many)
  File "/home/andrew/misc/marshmallow/marshmallow/schema.py", line 789, in _invoke_field_validators
    field_obj=field_obj
  File "/home/andrew/misc/marshmallow/marshmallow/marshalling.py", line 74, in call_and_store
    errors.setdefault(field_name, []).extend(err.messages)
AttributeError: 'dict' object has no attribute 'extend'