Move MailError to BoxError

src/boxerror.js

@@ -60,3 +60,9 @@ BoxError.UNKNOWN_ERROR = 'Unknown Error'; // only used for porting
 BoxError.prototype.toPlainObject = function () {
     return _.extend({}, { message: this.message, reason: this.reason }, this.details);
 };
+
+BoxError.prototype.setMessage = function (message) {
+    assert.strictEqual(typeof message, 'string');
+
+    this.message = message;
+};