Don't double dots in the end of encryption exception message

Currently we can generate doubled "dot"s in the end of encryption exception message here: https://github.com/mongodb/mongo-csharp-driver/blob/master/src/MongoDB.Driver/Encryption/MongoEncryptionException.cs#L32 if the original inner exception also is ended on dot. Which means that the final exception message can be similar to:

Encryption related exception: The remote certificate is invalid according to the validation procedure..