Fix TransactAsset Implementation

[apopiak]

This PR aims to fix the TransactAsset (tuple) implementation.
It converts local AssetNotFound errors into XcmError::AssetNotFound to allow the tuple implementation of TransactAsset to iterate properly. It also adjusts the other tuple implementations (notably withdraw_asset and deposit_asset) to have the same semantics.

Also adds some doc comments to XCM errors.

[gavofyork]

Would be good to add tests with this.

[apopiak]

As mentioned in chat, maybe we should remove the tuple implementation of TransactAsset to avoid the associated footguns?

[apopiak]

I adjusted the TransactAsset tuple implementation to something that I believe will be less error-prone.

[apopiak]

Would be good to add tests with this.

Do you mean integration tests?
This only shows up for the tuple implementation.

[apopiak]

@shawntabrizi Another issue I see with the Tuple impl is that the check_in and check_out implementations have no way of signalling to each other, so an asset might be checked in twice which might or might not be what you want.
(Of course your point still stands that people can write their own combinator that does the correct thing.)

[shawntabrizi]

Sounds like one bigger solution for all of this is a continue boolean which allows the user to directly control the behavior here

[apopiak]

@gavofyork Fine to merge?

[apopiak]

bot merge

[ghost]

Trying merge.