add helpful error to findDOMNode util, closes #147

lib/utils/find-dom-node.js

@@ -7,7 +7,15 @@
  */
 
 function findDOMNode(node) {
-  return window.document.querySelector(`[data-key="${node.key}"]`)
+  const el = window.document.querySelector(`[data-key="${node.key}"]`)
+
+  if (!el) {
+    throw new Error(`Unable to find a dom node for "${node.key}". This is
+often because of forgetting to add \`props.attributes\` to a component
+returned from \`renderNode\`.`)
+  }
+
+  return el
 }
 
 /**