Support ReturnValuesOnConditionCheckFailure

[damir-fell]

Hi!
Dynamodb recently started supporting ReturnValuesOnConditionCheckFailure which allows to show which values caused a conditional error trigger which hugely helps debugging issues with conditional writes. Can this be supported in PynamoDB?

https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_ConditionCheck.html

Thanks!

[damir-fell]

Hi @ikonst,
You think anybody will pick up on this or we should make a contribution?

[ikonst]

I think #1226 is slightly related but otherwise contributions are welcome.