fix: MongoError circular dependency warning (#2734)

Author: ztomm

lib/operations/operation.js

@@ -1,7 +1,7 @@
 'use strict';
 
 const Explain = require('../explain').Explain;
-const MongoError = require('../core').MongoError;
+const MongoError = require('../core/error').MongoError;
 
 const Aspect = {
   READ_OPERATION: Symbol('READ_OPERATION'),