fix: More explicit error message when DOM selector is invalid

Fixed #105

Author: pixelastic

lib/utils.js

@@ -1,13 +1,21 @@
 function getContainerNode(selectorOrHTMLElement) {
-  if (typeof selectorOrHTMLElement === 'string') {
-    return document.querySelector(selectorOrHTMLElement);
+  var isFromString = (typeof selectorOrHTMLElement === 'string');
+  var domElement;
+  if (isFromString) {
+    domElement = document.querySelector(selectorOrHTMLElement);
+  } else {
+    domElement = selectorOrHTMLElement;
   }
 
-  if (!isDomElement(selectorOrHTMLElement)) {
-    throw new Error('Container must be `string` or `HTMLElement`');
+  if (!isDomElement(domElement)) {
+    var errorMessage = 'Container must be `string` or `HTMLElement`.';
+    if (isFromString) {
+      errorMessage += ' Unable to find ' + selectorOrHTMLElement;
+    }
+    throw new Error(errorMessage);
   }
 
-  return selectorOrHTMLElement;
+  return domElement;
 }
 
 function isDomElement(o) {