Nest errors in pallet-xcm

[franciscoaguirre]

Errors in pallet-xcm are very bad. You execute something and all you get is LocalExecutionIncomplete. That's it, nothing else. You don't know if you had insufficient balance, if your account didn't convert, nothing.

One thing we can do to improve this is to nest the error from the executor inside of LocalExecutionIncomplete. That way, we can provide more information in the event of an error. Errors in FRAME are 4 bytes long and right now we're only using one of them. By using more bytes we can have nested enums that give the end user (or application) a better error.

Still, in order to make this really good, we would need to rework the errors themselves. For example, FailedToTransactAsset takes a string that we can't encode in less than 4 bytes. So we need to change that or convert that to a relevant enum that we can provide in the error.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/13546080272
Failed job name: fmt

[acatangiu]

This should stay stable, no change, otherwise breaks PJS decoding.

Just use new (index=25) error variant.