Use error message from DigitalOcean if available

src/dns/digitalocean.js

@@ -23,7 +23,7 @@ const assert = require('assert'),
 const DIGITALOCEAN_ENDPOINT = 'https://api.digitalocean.com';
 
 function formatError(response) {
-    return `DigitalOcean DNS error ${response.status} ${response.text}`;
+    return `DigitalOcean DNS error ${response.status} ${response.body ? response.body.message : response.text}`;
 }
 
 function removePrivateFields(domainObject) {