Write IBC ack error on failures

[sug0]

Problem statement

Right now, if an IBC transaction fails on Namada, especially due to VP errors, we do not emit any acknowledgement events to inform the counterparty. This leads to the funds only being recoverable on the source chain by waiting for the transfer to timeout, which in some cases (e.g. with Packet Forward Middleware retries) can take 10 minutes.

To considerably improve the UX, transfers (ideally) should fail immediately with an ack error being written to storage, accompanied by an event.

Proposed solutions

Define transaction epilogue

A generic solution could involve a tx epilogue, where additional logic can be executed at the end of VP execution.

[sug0]

/cc @yito88 @cwgoes

[yito88]

Essentially, this issue is that a VP execution can't submit an event in the current transaction execution model. We can't add other changes, including events, since any change is handled in the transaction execution, and event submission should also be checked by the VP.

Some errors can be caught in the execution by validating the IBC transfer in the transaction execution because ibc-rs can call the validation functions (just return Ok now) in the execution function. However, it can't resolve this issue when other VPs, such as the Multitoken VP and MASP VP, reject the transaction without incorporating the same validations of these VPs into their validation functions.

https://github.com/informalsystems/ibc-rs/blob/be82d123448a59ccea305c9e918f07aca5ec1a6f/ibc-apps/ics20-transfer/src/handler/on_recv_packet.rs#L54

https://github.com/informalsystems/ibc-rs/blob/be82d123448a59ccea305c9e918f07aca5ec1a6f/ibc-apps/ics20-transfer/src/handler/on_recv_packet.rs#L102

namada/crates/ibc/src/context/token_transfer.rs

Line 220 in a71fb62

fn unescrow_coins_validate(

namada/crates/ibc/src/context/token_transfer.rs

Line 231 in a71fb62

fn mint_coins_validate(