Better exception handling in threat_intel.opendns._cached_by_domain wrapper

[jjsendor]

The wrapper raises a very descriptive exception 'dang' when the response from OpenDNS Investigate API call is empty. We could do better than that.

[jjsendor]

Digging deeper into this issue, I have realized that in the call to OpenDNS Investigate Categorization endpoint, the data in the post request is actually the whole array of domains requested in the call to threat_intel.opendns.categorization(), rather than a list of smaller arrays containing up to max_requests elements.

[jjsendor]

Looking at the examples in the official documentation, it is actually ok to put multiple domains in the JSON array sent ot the endpoint, though there is still a hard limit of 1000 domain in the single request.

This must be the root cause for the failing calls to these API endpoint.

[jjsendor]

I will take care of fixing the calls to the Domain Categorization endpoint in #60.

Regarding 'dang' we really need some better exception handling here.