lib/string.gi: add \+ for strings, shows more helpful error

[fingolfin]

This is an alternative to PR #1313, which instead actually concatenates its inputs.

[codecov]

Codecov Report

Merging #1314 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1314      +/-   ##
==========================================
- Coverage    61.7%    61.7%   -0.01%     
==========================================
  Files         916      916              
  Lines      277105   277106       +1     
  Branches    15320    15321       +1     
==========================================
  Hits       170985   170985              
- Misses     102128   102130       +2     
+ Partials     3992     3991       -1

Impacted Files	Coverage Δ
lib/string.gi	39.45% <0%> (-0.06%)	⬇️
src/hpc/threadapi.c	28.27% <0%> (-0.29%)	⬇️
hpcgap/src/vars.c	40% <0%> (+0.09%)	⬆️
src/funcs.c	63.56% <0%> (+0.27%)	⬆️

[olexandr-konovalov]

At least this alternative seems uncontroversial and makes GAP more user-friendly.

[ChrisJefferson]

Maybe add some tests to this, just for sanity, then I think we should merge it.

[olexandr-konovalov]

Indeed, adding a test as @ChrisJefferson suggests may have sense (for example, if a package will appear, which installs a method for \+ to concatenate strings). After that I suggest to merge this a solution - it does not change existing functionality, but makes GAP more user-friendly.

[fingolfin]

I will not add tests to this, but anybody else is of course welcome to