Closes #665 - Correctly handle user not granting auth permissions, display friendly error

shared/utils/getQueryVariable.js

@@ -0,0 +1,12 @@
+// @flow
+
+export default function getQueryVariable(variable: string) {
+  var query = window.location.search.substring(1);
+  var vars = query.split('&');
+  for (var i = 0; i < vars.length; i++) {
+    var pair = vars[i].split('=');
+    if (pair[0] == variable) {
+      return pair[1];
+    }
+  }
+}