Change HANDLE_METHOD_NOT_FOUND to never return

[fingolfin]

Using return; to exit a "method not found" break loop had more or less the same effect as using quit;

This PR contains PR #2449, but the second commit in it may be more controversial (?) than the fix in the first PR; also, only that fix should be backported to stable-4.9, not the second commit in here; hence I have put this into a separate PR.q

[codecov]

Codecov Report

Merging #2450 into master will decrease coverage by <.01%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #2450      +/-   ##
==========================================
- Coverage   73.95%   73.95%   -0.01%     
==========================================
  Files         484      484              
  Lines      245403   245402       -1     
==========================================
- Hits       181482   181475       -7     
- Misses      63921    63927       +6

Impacted Files	Coverage Δ
lib/methsel2.g	43.93% <100%> (ø)	⬆️
src/opers.c	94.48% <83.33%> (-0.14%)	⬇️
hpcgap/lib/hpc/stdtasks.g	63.42% <0%> (-0.77%)	⬇️
src/stats.c	87.05% <0%> (-0.14%)	⬇️