Revset: allow returning errors during iteration #15172

Job	Run time
Build jj-lib without Git support	46s
Check protos	31s
build (ubuntu-latest)	4m 47s
cargo-deny (advisories)	24s
Clippy check	2m 36s
cargo-deny (bans licenses sources)	25s
Check that MkDocs can build the docs	20s
Check that MkDocs can build the docs with Poetry 1.8	23s
build (macos-13)	7m 20s
Check formatting	15s
build (macos-14)	3m 27s
build (windows-latest)	9m 5s
build (ubuntu-latest, --all-features)	5m 41s
	36m 0s

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