[xcm] Failing to deposit dust (below minimum value) will fail the whole XCM program

[acatangiu]

Description

The implementation of DepositAsset will directly abort if any asset can't be deposited. This can happen if the amount of the asset is below its minimum balance and the receiver doesn't have a balance of it yet, in that case the asset transactor will directly fail with TokenError::BelowMinimum via the following call flow:

• polkadot-sdk/polkadot/xcm/xcm-executor/src/lib.rs

  Line 625 in ac3773e

  Config::AssetTransactor::deposit_asset(

• polkadot-sdk/polkadot/xcm/xcm-builder/src/fungibles_adapter.rs

  Line 390 in 3dece31

  >::deposit_asset(what, who, context)

• polkadot-sdk/polkadot/xcm/xcm-builder/src/fungibles_adapter.rs

  Line 291 in 3dece31

  Assets::mint_into(asset_id, &who, amount)

• polkadot-sdk/substrate/frame/support/src/traits/tokens/fungibles/regular.rs

  Line 264 in bdbe982

  let actual = Self::increase_balance(asset.clone(), who, amount, Exact)?;

• polkadot-sdk/substrate/frame/support/src/traits/tokens/fungibles/regular.rs

  Line 231 in bdbe982

  Err(TokenError::BelowMinimum.into())

This is especially a problem for more complex XCMs that transfer multiple assets, which can completely fail when the leftover fee-asset amount (after paying the fees) is just dust and might not meet "min value" for depositing.

Proposed solution

Treat TokenError::BelowMinimum error as non-fatal in xcm-executor, effectively ignore errors of failures to deposit dust.

[acatangiu]

Also related #901