How to retrieve MongoDb collection validator rules?

On MongoDB 3.4.4 I've created a collection with a validator, but now some inserts fail this rules and I can't understand why.

1. Is there a way to output the rules of the validor? I'm afraid the rules applied are different from what I think they are...
2. Is there a way to improve the error message? "Document failed validation" in this scenario is quite useless.

Thank you!

1. You can see the validation rules (among other collection information) with db.getCollectionInfos() for all collections or db.getCollectionInfos({name: "myCollection"}) for a specific collection: MongoDB docs

2. Have a look at this answer.