minimal set of changes for returning failure to find non-zero pivot in FFGJ #324

Job	Run time
build (Debug, yes, 3.12, yes, ubuntu-20.04, gcc)	3m 16s
build (Debug, yes, 3.11, yes, ubuntu-20.04, gcc)	3m 15s
build (Debug, yes, 3.10, yes, ubuntu-20.04, gcc)	3m 12s
build (Debug, yes, 3.9, yes, ubuntu-20.04, gcc)	3m 11s
build (Release, 3.8, yes, ubuntu-20.04, gcc)	3m 21s
build (Release, 3.8, yes, gmpxx, no, ubuntu-20.04, gcc)	3m 28s
build (Release, 3.8, yes, ubuntu-20.04, gcc)	3m 30s
build (Release, yes, 3.8, ubuntu-20.04, gcc)	3m 32s
build (Release, 3.9, yes, ubuntu-20.04, gcc)	3m 33s
build (Release, 3.9, yes, flint, yes, ubuntu-20.04, gcc)	3m 40s
build (Debug, 3.8, yes, yes, ubuntu-20.04, clang)	3m 7s
build (Release, 3.8, yes, ubuntu-20.04, clang)	3m 27s
build (Debug, 3.10, yes, 14, yes, boostmp, -k "not integer_nthroot", ubuntu-22.04, deb http://apt...	5m 10s
build (Debug, 3.8, yes, 5, macos-latest, clang)	9m 23s
build (Release, 3.9, no, macos-latest, clang)	9m 1s
build (Debug, 3.8, no, macos-latest, gcc)	7m 15s
build (Release, 3.8, macos-latest, gcc)	7m 59s
build (Release, 3.11, ubuntu-20.04, yes, yes, yes, yes, yes, yes, flint, yes, gcc)	3m 46s
	1h 23m 6s

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