handle non-json non quandl error responses

[ccleung]

Tracker Ticket

https://quandl.atlassian.net/browse/AP-1808

Description

• For non-standard error responses, e.g., non-json or non quandl_error responses (responses that do not have the root key quandl_error in json format), show a generic error message to users
• NOTE: this will also probably cover the case where the response body is empty - as we will encounter a json parse error (though i was not able to repro a situation where the response body was empty)

  Note to the reviewer

• see comments here: https://quandl.atlassian.net/browse/AP-1861 on how non standard response body may come about

  Note to QA

  Dependency

• None

[junos]

code looks good to me. 👍