minimal set of changes for returning failure to find non-zero pivot in FFGJ #390

Job	Run time
build (Debug, yes, 3.10, yes, ubuntu-20.04, gcc)	1m 21s
build (Release, 3.13, yes, ubuntu-20.04, gcc)	1m 20s
build (Debug, yes, 3.11, yes, ubuntu-20.04, gcc)	1s
build (Release, 3.13, yes, ubuntu-20.04, gcc)	2m 49s
build (Debug, yes, 3.9, yes, ubuntu-20.04, gcc)	1s
build (Release, 3.13, yes, gmpxx, no, ubuntu-20.04, gcc)	3m 14s
build (Debug, 3.13, yes, 5, macos-13, clang)	5m 41s
build (Release, yes, 3.13, ubuntu-20.04, gcc)	3m 20s
build (Release, 3.9, no, macos-13, clang)	5m 25s
build (Release, 3.9, yes, ubuntu-20.04, gcc)	1s
build (Release, 3.9, yes, flint, yes, ubuntu-20.04, gcc)	1s
build (Debug, yes, 3.12, yes, ubuntu-20.04, gcc)	3m 17s
build (Debug, 3.10, yes, 14, yes, boostmp, -k "not integer_nthroot", ubuntu-22.04, deb http://apt...	4m 57s
build (Debug, 3.13, yes, yes, ubuntu-20.04, clang)	3m 19s
build (Release, 3.13, yes, ubuntu-20.04, clang)	3m 21s
build (Debug, 3.13, no, macos-13, gcc)	4m 48s
build (Release, 3.13, macos-13, gcc)	5m 9s
build (Release, 3.11, ubuntu-20.04, yes, yes, yes, yes, yes, yes, flint, yes, gcc)	3m 18s
	51m 23s

Provide feedback