fix: save execution stack in query as string

Author: durran

lib/query.js

@@ -4433,10 +4433,10 @@ Query.prototype.exec = async function exec(op) {
       str = str.slice(0, 60) + '...';
     }
     const err = new MongooseError('Query was already executed: ' + str);
-    err.originalStack = this._executionStack.stack;
+    err.originalStack = this._executionStack;
     throw err;
   } else {
-    this._executionStack = new Error();
+    this._executionStack = new Error().stack;
   }
 
   let skipWrappedFunction = null;