chore: fix clippy::question-mark lint in execute_effects

[shsw228]

The Lint job currently fails on main with a single pre-existing
clippy::question-mark error in yashiki/src/app/effects.rs. This replaces the
match with ?, which also matches the neighbouring Effect::ExecCommand arm.

Behaviour is unchanged: exec_command_tracked and execute_effects both use
String as the error type, so ? propagates exactly as return Err(e) did.

Verified with the CI commands: cargo test --all (310 passed),
cargo fmt --all --check, cargo clippy --all -- -D warnings.