Define nonexisting DOMError for Safari

Fix JIO crash when running inside Safari because of non-existing DOMError object

/reviewed-on nexedi/jio!47

src/jio.js

@@ -10,6 +10,11 @@
     };
   }
 
+  /* Safari does not define DOMError */
+  if (window.DOMError === undefined) {
+    window.DOMError = {};
+  }
+
   var util = {},
     jIO;