Include an error code for each response

[gldraphael]

This PR updates the output to include a new error field for each item in responses[].
The field will be null if no error exists for that response.

Closes #28

Formatted response when querying an existing domain with cargo run -- --json dns.lookup.dog:

{
   "responses":[
      {
         "queries":[
            {
               "name":"dns.lookup.dog.",
               "class":"IN",
               "type":"A"
            }
         ],
         "answers":[
            {
               "name":"dns.lookup.dog.",
               "class":"IN",
               "ttl":0,
               "type":"A",
               "data":{
                  "address":"51.159.146.31"
               }
            },
            {
               "name":"dns.lookup.dog.",
               "class":"IN",
               "ttl":0,
               "type":"A",
               "data":{
                  "address":"51.159.149.163"
               }
            }
         ],
         "authorities":[],
         "additionals":[],
         "error":null
      }
   ]
}

Formatted response when querying a non-existent domain with cargo run -- --json dns.lookup123.dog:

{
   "responses":[
      {
         "queries":[
            {
               "name":"dns.lookup123.dog.",
               "class":"IN",
               "type":"A"
            }
         ],
         "answers":[],
         "authorities":[
            {
               "name":"dog.",
               "class":"IN",
               "ttl":1800,
               "type":"SOA",
               "data":{
                  "mname":"v0n0.nic.dog.",
                  "rname":"hostmaster.donuts.email.",
                  "serial":1714974458,
                  "refresh_interval":7200,
                  "retry_interval":900,
                  "expire_limit":1209600,
                  "minimum_ttl":3600
               }
            }
         ],
         "additionals":[],
         "error":"NXDomain"
      }
   ]
}

[Dj-Codeman]

Looks good! I'll write a few tests for the errors before the release but otherwise this is good.