Merge pull request #5199 from jimfb/number-found

Print number of matches (previously unclear if zero or multiple)

src/test/ReactTestUtils.js

@@ -237,7 +237,9 @@ var ReactTestUtils = {
   findRenderedDOMComponentWithTag: function(root, tagName) {
     var all = ReactTestUtils.scryRenderedDOMComponentsWithTag(root, tagName);
     if (all.length !== 1) {
-      throw new Error('Did not find exactly one match for tag:' + tagName);
+      throw new Error('Did not find exactly one match ' +
+        '(found ' + all.length + ') for tag:' + tagName
+      );
     }
     return all[0];
   },