External merge tools: better error message for unsupported usage #10873

Job	Run time
build (ubuntu-latest)	3m 48s
Check protos	57s
Check formatting	17s
build (macos-14)	3m 32s
Check that MkDocs can build the docs	25s
build (windows-latest)	8m 31s
build (ubuntu-latest, --all-features)	4m 43s
cargo-deny (advisories)	23s
cargo-deny (bans licenses sources)	28s
Clippy check	2m 33s
Check that MkDocs can build the docs with Poetry 1.3.2	33s
	26m 10s

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