Warn and continue execution upon upsert failure

Currently, a failure to upsert to KV fails the whole COPY TO operation, although what has already been written to KV persists.

This ticket is to track changing this behavior to warn in case of failure, and continue execution.

This behavior should be only for data related failures, for example, key is not a string, or key is larger than maximum size, but a complete operation abort should take place for failures such as timing out, target host is down, ... etc.