Introduce Shopify.Error to handle response failures

[vladimir-e]

@Ninigi thanks for a quick feedback on #37

I think returning {:error, %Shopify.Error{}} would bubble up, so guess your approach would work 👍
We would have to put some thought into the error fields, since we might want to use it for the errors returned by the shopify api as well

I think we should keep returning shopify errors wrapped in Shopify.Response, because it is response.

And for failed requests we pass errors from HTTPoison. Actually HTTPoison does the same, passing errors from hackney. So with the solution I propose in this PR we get ability to handle these errors coming from the deep.

we would need a data and a code field to make it backwards compatible.

If we don't use Shopify.Error for API errors we don't really need to make it backward compatible.

I tested this solution on my real app that exports thousands of records, by switching network on and off to produce network errors, and it worked well.

[Ninigi]

Thanks, will look into this on Sunday :)

[Ninigi]

closes #37