Refactored AxiosError to a constructor;

Refactored `Cancel` to a constructor, a subclass of the `AxiosError`;
Expose CanceledError class;
Refactored axios error codes;
Added `toFlatObject` util;

lib/core/settle.js

@@ -1,6 +1,6 @@
 'use strict';
 
-var createError = require('./createError');
+var AxiosError = require('./AxiosError');
 
 /**
  * Resolve or reject a Promise based on response status.
@@ -14,10 +14,10 @@ module.exports = function settle(resolve, reject, response) {
   if (!response.status || !validateStatus || validateStatus(response.status)) {
     resolve(response);
   } else {
-    reject(createError(
+    reject(new AxiosError(
       'Request failed with status code ' + response.status,
+      [AxiosError.ERR_BAD_REQUEST, AxiosError.ERR_BAD_RESPONSE][Math.floor(response.status / 100) - 4],
       response.config,
-      null,
       response.request,
       response
     ));