Detect if DOMException exists via typeof instead of trying to call it and...

Detect if DOMException exists via typeof instead of trying to call it and catching the exception which may get thrown

fixes https://github.com/github/fetch/issues/724

fetch.js

@@ -455,9 +455,8 @@ Response.redirect = function(url, status) {
 }
 
 export var DOMException = global.DOMException
-try {
-  new DOMException()
-} catch (err) {
+
+if (typeof DOMException !== 'function') {
   DOMException = function(message, name) {
     this.message = message
     this.name = name