Return status for HTTP errors in errors extensions

[pbostrom]

PR to return HTTP status codes as discussed in #27.
When an HTTP error occurs, re-graph will insert the HTTP status into the response according to 3 conditions:

• Response contains a valid GraphQL errors map: update the map with HTTP status
• Response is a map but does not contain a valid errors map: merge in default errors map
• Response is anything else: return default errors map

where the default errors map is:

{:errors [{:message "The HTTP call failed."
           :extensions {:status status}}]}

4 test cases were added

• Query error with invalid graphql response (string body)
• Query error with invalid graphql response (map body)
• Query error with valid graphql error response
• No query error