Improve ergonomics of `delete` op

[kuy]

Is your feature request related to a problem? Please describe.

delete op return Ok even if given key is not found. This is not a bug, it's an idempotent operation. That said, I was surprised really.

Describe the solution you'd like

Some options:

• rename it to delete_if_exist
• return an error if not exist
• hybrid of above two: provide delete_if_exist op and delete op which return an error

Describe alternatives you've considered

N/A

Additional context

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