Raise exception on unrecognized Content-Type

[rattrayalex]

https://github.com/graphql-python/graphql-django-view/blob/master/graphql_django_view/__init__.py#L93 will tell the user:

<HttpResponseBadRequest status_code=400, "application/json">
b'{"errors":[{"message":"Must provide query string."}]}'

if the content-type is unexpected. An informative exception should be raised instead, eg:

raise HttpError(HttpResponseBadRequest('Unrecognized content-type: text/plain. Please send one of [application/graphql, application/json, application/x-www-form-urlencoded, multipart/form-data]'))

[syrusakbary]

Makes sense!