Merge pull request #15420 from Automattic/vkarpov15/gh-15416

feat(error): set `cause` to MongoDB error `reason` on ServerSelection errors

Author: vkarpov15

lib/error/serverSelection.js

@@ -49,6 +49,7 @@ class MongooseServerSelectionError extends MongooseError {
         this[key] = err[key];
       }
     }
+    this.cause = reason;
 
     return this;
   }