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/enhanceError.js

@@ -1,43 +0,0 @@
-'use strict';
-
-/**
- * Update an Error with the specified config, error code, and response.
- *
- * @param {Error} error The error to update.
- * @param {Object} config The config.
- * @param {string} [code] The error code (for example, 'ECONNABORTED').
- * @param {Object} [request] The request.
- * @param {Object} [response] The response.
- * @returns {Error} The error.
- */
-module.exports = function enhanceError(error, config, code, request, response) {
-  error.config = config;
-  if (code) {
-    error.code = code;
-  }
-
-  error.request = request;
-  error.response = response;
-  error.isAxiosError = true;
-
-  error.toJSON = function toJSON() {
-    return {
-      // Standard
-      message: this.message,
-      name: this.name,
-      // Microsoft
-      description: this.description,
-      number: this.number,
-      // Mozilla
-      fileName: this.fileName,
-      lineNumber: this.lineNumber,
-      columnNumber: this.columnNumber,
-      stack: this.stack,
-      // Axios
-      config: this.config,
-      code: this.code,
-      status: this.response && this.response.status ? this.response.status : null
-    };
-  };
-  return error;
-};