make the `execpools/*` schedulers conditionally infallible #2733

Job	Run time
CPU (Windows) (msvc, Release)	15m 2s
macos-CPU (MacOS) (clang, Debug)	4m 14s
macos-CPU (MacOS) (clang, Release)	3m 35s
CPU (Windows) (msvc, Debug)	12m 32s
CPU (gcc 11, Release, ASAN)	2m 28s
CPU (gcc 11, Release)	1m 51s
CPU (clang 16, Release, ASAN)	2m 26s
CPU (gcc 13, Debug)	11m 40s
CPU (gcc 12, Release, TSAN)	2m 24s
CPU (gcc 11, Debug)	1m 51s
CPU (clang 16, Debug)	2m 38s
CPU (clang 16, Release)	2m 17s
CI (CPU) (MacOS)	4s
CI (CPU)	2s
CI (CPU) (Windows)	7s
	1h 3m 11s

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