Nest errors in pallet-xcm

[franciscoaguirre]

Errors in pallet-xcm are frustrating. You execute an XCM instruction and get LocalExecutionIncomplete: no details, no context. Was it insufficient balance? An account conversion failure? You're left guessing.

To fix this, we introduce LocalExecutionIncompleteWithError, which nests an ExecutionError enum inside LocalExecutionIncomplete. This provides specific error details, like insufficient balance or asset transaction failures, making debugging easier for users and applications. FRAME errors are limited to 4 bytes, and we were only using one. By nesting ExecutionError, we utilize the full 4 bytes with a compact enum that mirrors XcmError for consistency.

To make this robust, we reworked problematic errors. For instance, FailedToTransactAsset previously took a string, which couldn’t fit in 4 bytes. We’ve replaced it with a unit variant in ExecutionError, logging the string for debugging while keeping the encoding compact. This ensures detailed, efficient error reporting.

[acatangiu]

Either nest or do smth like Error("description") and each layer along the way adds to "description", but nesting should work better

[raymondkfcheung]

/cmd prdoc --audience runtime_dev --bump patch

[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/15356104242
Failed job name: test-linux-stable-no-try-runtime