Unparsable API responses should raise UnavailableError

[mtowers]

When the API returns a response that is not valid JSON, the service should be presumed to be unavailable.

Only when valid JSON is returned that represents a server-side error should api_request return a ruby hash with a valid SailThru error code assigned.

See related pull request.

[lindsaycooper]

@mtowers This change is not backwards compatible. We will consider making this change in a major version upgrade but for now we are going to keep it as is.