nixd/lib/Controller: use `CheckReturn` to simplify early-returns (NFC) #3260

Job	Run time
nix-build-develop-clang (none, release, true)	2m 53s
nix-build-develop-gcc (none, release, true)	3m 5s
nix-build-develop-clang (none, release, false)	2m 57s
nix-build-develop-gcc (none, release, false)	3m 6s
nix-build-develop-clang (none, debug, true)	2m 46s
nix-build-develop-gcc (none, debug, true)	2m 57s
nix-build-develop-clang (none, debug, false)	2m 40s
nix-build-develop-gcc (none, debug, false)	3m 5s
nix-build-develop-gcc (address, release, true)	4m 39s
nix-build-develop-gcc (address, release, false)	4m 47s
nix-build-develop-gcc (address, debug, true)	3m 58s
nix-build-develop-gcc (address, debug, false)	4m 12s
nix-build-develop-gcc (undefined, release, true)	3m 57s
nix-build-develop-gcc (undefined, release, false)	4m 5s
nix-build-develop-gcc (undefined, debug, true)	3m 16s
nix-build-develop-gcc (undefined, debug, false)	3m 15s
	55m 38s

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