Error reporting POST to Product error

从 xologic_connection/product.py 第 692 行开始，沿着报告中显示的 pyactiveresource 堆栈追踪保存和错误处理路径。重现无效 product 或重复 variant 的 422 响应，然后验证是否会公开底层的 Shopify 验证消息，而不是 AttributeError。

When there is an error in the product or variants that are sent via post to the Product endpoint, the error message is structured in a way that the underlying pyactiveresource.connection.ResourceInvalid object cannot decode it properly, which leads to a very un-informative error.
Please see stacktrace below:

Traceback (most recent call last):
  File "C:\Users\user\Dropbox\virtualenvs\lighting\lib\site-packages\pyactiveresource\connection.py", line 286, in _open
    http_response = self._handle_error(self._urlopen(request))
  File "C:\Users\user\Dropbox\virtualenvs\lighting\lib\site-packages\pyactiveresource\connection.py", line 318, in _urlopen
    return urllib.request.urlopen(request)
  File "C:\Python34\lib\urllib\request.py", line 161, in urlopen
    return opener.open(url, data, timeout)
  File "C:\Python34\lib\urllib\request.py", line 469, in open
    response = meth(req, response)
  File "C:\Python34\lib\urllib\request.py", line 579, in http_response
    'http', request, response, code, msg, hdrs)
  File "C:\Python34\lib\urllib\request.py", line 507, in error
    return self._call_chain(*args)
  File "C:\Python34\lib\urllib\request.py", line 441, in _call_chain
    result = func(*args)
  File "C:\Python34\lib\urllib\request.py", line 587, in http_error_default
    raise HTTPError(req.full_url, code, msg, hdrs, fp)
urllib.error.HTTPError: HTTP Error 422: Unprocessable Entity

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\user\Dropbox\virtualenvs\lighting\lib\site-packages\pyactiveresource\activeresource.py", line 824, in save
    data=self.encode())
  File "C:\Users\user\Dropbox\virtualenvs\lighting\lib\site-packages\pyactiveresource\connection.py", line 364, in post
    return self._open('POST', path, headers=headers, data=data)
  File "C:\Users\user\Dropbox\virtualenvs\lighting\lib\site-packages\shopify\base.py", line 23, in _open
    self.response = super(ShopifyConnection, self)._open(*args, **kwargs)
  File "C:\Users\user\Dropbox\virtualenvs\lighting\lib\site-packages\pyactiveresource\connection.py", line 288, in _open
    http_response = self._handle_error(err)
  File "C:\Users\user\Dropbox\virtualenvs\lighting\lib\site-packages\pyactiveresource\connection.py", line 414, in _handle_error
    raise ResourceInvalid(err)
pyactiveresource.connection.ResourceInvalid: Response(code=422, body="b'{"errors":{"base":["The variant \'Antique Bronze \\/ Ice pattern\' already exists. Please change at least one option value."]}}'", headers={'X-XSS-Protection': '1; mode=block; report=/xss-report?source%5Baction%5D=create&source%', 'X-Frame-Options': 'DENY', 'X-Stats-UserId': '0', 'X-Content-Type-Options': 'nosniff', , 'Date': 'Wed, 06 Sep 2017 17:17:23 GMT', 'Strict-Transport-Security': 'max-age=7776000', 'X-Stats-ApiPermissionId': '42905249', 'X-Sorting-Hat-PodId-Cached': '0', 'Content-Security-Policy': "default-src 'self' data: blob: 'unsafe-inline' 'unsafe-eval' https://* shopify-pos://*; block-all-mixed-content; child-src 'self' https://* shopify-pos://*; connect-src 'self' wss://* https://*; script-src https://cdn.shopify.com https://checkout.shopifycs.com https://js-agent.newrelic.com https://bam.nr-data.net https://dme0ih8comzn4.cloudfront.net https://api.stripe.com https://mpsnare.iesnare.com https://appcenter.intuit.com https://www.paypal.com https://maps.googleapis.com https://stats.g.doubleclick.net https://www.google-analytics.com https://visitors.shopify.com https://v.shopify.com https://widget.intercom.io https://js.intercomcdn.com 'self' 'unsafe-inline' 'unsafe-eval'; upgrade-insecure-requests; report-uri /csp-report?source%5Baction%5D=create&source%5Bapp%5D=Shopify&source%5Bcontroller%5D=admin%2Fproducts&source%5Bsection%5D=admin_api&source%5Buuid%5D=5e3df701-e2e3-4b70-bda3-b7d15592a0f5", 'X-Sorting-Hat-ShopId-Cached': '0', 'Connection': 'close', 'Referrer-Policy': 'origin-when-cross-origin', 'X-Request-Id': '5e3df701-e2e3-4b70-bda3-b7d15592a0f5', 'X-Shopify-Shop-Api-Call-Limit': '1/40', 'Content-Type': 'application/json; charset=utf-8', 'Server': 'nginx', 'X-ShardId': '15', 'X-Dc': 'ash', 'Transfer-Encoding': 'chunked', 'X-Download-Options': 'noopen', 'X-Permitted-Cross-Domain-Policies': 'none', 'HTTP_X_SHOPIFY_SHOP_API_CALL_LIMIT': '1/40', 'X-Sorting-Hat-PodId': '15', 'X-Sorting-Hat-Section': 'pod'}, msg="Unprocessable Entity")

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\user\Dropbox\virtualenvs\lighting\xologic_connection\product.py", line 692, in create_on_shopify
    success = new_product.save()
  File "C:\Users\user\Dropbox\virtualenvs\lighting\lib\site-packages\pyactiveresource\activeresource.py", line 832, in save
    self.errors.from_json(err.response.body)
AttributeError: 'list' object has no attribute 'from_json'
```
The relevant error is 
`body="b'{"errors":{"base":["The variant \'Antique Bronze \\/ Ice pattern\' already exists. Please change at least one option value."]}}'", `

It would be nice to get just that.