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/cancel/CanceledError.js

@@ -0,0 +1,22 @@
+'use strict';
+
+var AxiosError = require('../core/AxiosError');
+var utils = require('../utils');
+
+/**
+ * A `CanceledError` is an object that is thrown when an operation is canceled.
+ *
+ * @class
+ * @param {string=} message The message.
+ */
+function CanceledError(message) {
+  // eslint-disable-next-line no-eq-null,eqeqeq
+  AxiosError.call(this, message == null ? 'canceled' : message, AxiosError.ERR_CANCELED);
+  this.name = 'CanceledError';
+}
+
+utils.inherits(CanceledError, AxiosError, {
+  __CANCEL__: true
+});
+
+module.exports = CanceledError;