minimal set of changes for returning failure to find non-zero pivot in FFGJ #284

Job	Run time
build (Debug, yes, 3.12, yes, ubuntu-20.04, gcc)	3m 14s
build (Debug, yes, 3.11, yes, ubuntu-20.04, gcc)	3m 25s
build (Debug, yes, 3.10, yes, ubuntu-20.04, gcc)	3m 35s
build (Debug, yes, 3.9, yes, ubuntu-20.04, gcc)	3m 33s
build (Release, 3.8, yes, ubuntu-20.04, gcc)	3m 34s
build (Release, 3.8, yes, gmpxx, no, ubuntu-20.04, gcc)	3m 58s
build (Release, 3.8, yes, ubuntu-20.04, gcc)	3m 40s
build (Release, yes, 3.8, ubuntu-20.04, gcc)	3m 36s
build (Release, 3.9, yes, ubuntu-20.04, gcc)	3m 43s
build (Release, 3.9, yes, flint, yes, ubuntu-20.04, gcc)	3m 47s
build (Debug, 3.8, yes, yes, ubuntu-20.04, clang)	3m 32s
build (Release, 3.8, yes, ubuntu-20.04, clang)	3m 30s
build (Debug, 3.10, yes, 14, yes, boostmp, -k "not integer_nthroot", ubuntu-22.04, deb http://apt...	6m 43s
build (Debug, 3.8, yes, 5, macos-latest, clang)	6m 53s
build (Release, 3.9, no, macos-latest, clang)	11m 24s
build (Debug, 3.8, no, macos-latest, gcc)	5m 36s
build (Release, 3.8, macos-latest, gcc)	6m 13s
build (Release, 3.8, ubuntu-20.04, yes, yes, yes, yes, yes, flint, yes, gcc)	3m 45s
	1h 23m 41s

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