findAndModify should only return false if there is an error, not missing doc. findAndRemove throws an exception when no record found

findAndRemove throws CommandResult.Failure when no record in the collection found. Problems:

1. Java driver 2.4 returned null in this scenario
2. The exception has code=-5 which makes it indistinguishable from real errors.

Test case: https://github.com/alaz/mongo-java-driver/commit/56478bbefdd99fb97a564a449e99772efc08e7b8