`get_packet_{commitment,acknowledgement}` should return error if `delete_packet_{commitment,acknowledgement}` called

[rnbguy]

Currently, get_packet_commitment returns the commitment stored at a commitment path. But this commitment may not be valid - as delete_packet_commitment may have set it to empty.

This leads to transaction failure at ibc-rs implementation, where it should have succeeded. The false failures due to get_packet_commitment at ibc-rs,

• timeout_on_close.rs
• acknowledgement.rs
• acknowledgement.rs

The argument for get_packet_acknowledgement and delete_packet_acknowledgement is the same. The false failure due to get_packet_acknowledgement at ibc-rs,

• recv_packet.rs