UnicodeDecodeError in format_error

[Dmytro Vdovychynskyi]

Greetings!
After upgrading from

graphene                           2.0.1
graphene-django                    2.0.0
graphql-core                       2.0

to

graphene                           2.1.3
graphene-django                    2.2.0
graphql-core                       2.1

on python 2 I faced with UnicodeDecodeError: 'ascii' codec can't decode byte 0xd0 in position 0: ordinal not in range(128) issue when execute smth like

from graphene_django.views import GraphQLView

GraphQLView.format_error(error)

when error has non-ASCII characters like Ошибка

And seems like backward incompatibility appears after this change(found few similar issues on GH with six.text_type)

If you agree that this is a bug I will try to provide fix for it :)

[Dmytro Vdovychynskyi]

close this cause fixed via #223