Require non-zero coefficient ring in universal polynomial ring

[lgoettgens]

Progress towards #1856, split off from #1857.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.06%. Comparing base (9ebc781) to head (9fd6601).
Report is 5 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1858   +/-   ##
=======================================
  Coverage   88.06%   88.06%           
=======================================
  Files         120      120           
  Lines       30057    30058    +1     
=======================================
+ Hits        26469    26472    +3     
+ Misses       3588     3586    -2     

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[fingolfin]

Having a period at the end is somewhat rare in our error messages. A quick experiment in Oscar.jl:

$ git grep -E 'error\(.*\."' | wc -l
     101
$ git grep -E 'error\(' | wc -l
    1526

and

$ git grep -E '@req.*\."' | wc -l
     137
$ git grep -E '@req.*"' | wc -l
    1599

So less than 10% of our @req calls use a trailing period. The numbers for error` are a bit less reliably because they miss the case were the string spans over more than one line but I don't think there will be a fundamentally different outcome.

Anyway I am merging this and the other PRs now as this is a very minor point and also a matter of taste.