nixd/Server/format: suppress error popups #1720

Job	Run time
nix-build-develop-clang (none, release, true)	2m 59s
nix-build-develop-gcc (none, release, true)	3m 11s
nix-build-develop-clang (none, release, false)	4m 13s
nix-build-develop-gcc (none, release, false)	6m 5s
nix-build-develop-clang (none, debug, true)	2m 42s
nix-build-develop-gcc (none, debug, true)	5m 22s
nix-build-develop-clang (none, debug, false)	4m 35s
nix-build-develop-gcc (none, debug, false)	2m 59s
nix-build-develop-gcc (undefined, release, true)	6m 46s
nix-build-develop-gcc (undefined, release, false)	7m 7s
nix-build-develop-gcc (undefined, debug, true)	3m 33s
nix-build-develop-gcc (undefined, debug, false)	3m 34s
	53m 6s

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