Fix XCM Barrier Rejection Handling to Return Incomplete with Weight (#7843)

This PR addresses an issue with the handling of message execution when
blocked by the barrier. Instead of returning an `Outcome::Error`, we
modify the behaviour to return `Outcome::Incomplete`, which includes the
weight consumed up to the point of rejection and the error that caused
the blockage.

This change ensures more accurate weight tracking during message
execution, even when interrupted. It improves resource management and
aligns the XCM executor’s behaviour with better error handling
practices.

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Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>