Invalid use of / on ZmodnZObjs (e.g. dividing a unit by a zero divisor) produces an error, instead of returning fail

[fingolfin]

If you want fail, use Quotient, which was made for that.

[DominikBernhardt]

@fingolfin I think you forgot to adjust a few tests as they still expect fail as an output.

[codecov]

Codecov Report

Merging #3610 into master will decrease coverage by 9.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #3610      +/-   ##
==========================================
- Coverage   84.64%    75.6%   -9.05%     
==========================================
  Files         698      646      -52     
  Lines      345549   296727   -48822     
==========================================
- Hits       292482   224328   -68154     
- Misses      53067    72399   +19332

Impacted Files	Coverage Δ
lib/zmodnz.gi	78.76% <0%> (-19.43%)	⬇️
src/fibhash.h	0% <0%> (-100%)	⬇️
src/syntaxtree.c	5.44% <0%> (-90.72%)	⬇️
lib/meatauto.gi	5.99% <0%> (-89.32%)	⬇️
lib/nilpquot.gi	11.11% <0%> (-88.89%)	⬇️
src/modules.h	16.66% <0%> (-83.34%)	⬇️
lib/helpt2t.gi	0.23% <0%> (-83.14%)	⬇️
src/dteval.c	3.3% <0%> (-80.12%)	⬇️
src/objset.c	8.13% <0%> (-76.74%)	⬇️
lib/dt.g	7.75% <0%> (-72.2%)	⬇️
... and 422 more

[coveralls]

Coverage remained the same at 84.395% when pulling 9101aa9 on fingolfin:mh/zmodnz-strict-div into 659250f on gap-system:master.