Improve error reporting for rep0, repAs0

[johnynek]

give rep0/repAs0 the same treatment as #389

[johnynek]

cc @carueda

[codecov-commenter]

Codecov Report

Merging #396 (615c6b6) into main (c52f1f6) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #396      +/-   ##
==========================================
- Coverage   96.65%   96.63%   -0.02%     
==========================================
  Files           9        9              
  Lines        1284     1278       -6     
  Branches      124      115       -9     
==========================================
- Hits         1241     1235       -6     
  Misses         43       43              

Impacted Files	Coverage Δ
core/shared/src/main/scala/cats/parse/Parser.scala	96.31% <100.00%> (-0.12%)	⬇️
...shared/src/main/scala/cats/parse/Accumulator.scala	100.00% <0.00%> (+2.43%)	⬆️

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[johnynek]

in the presence of the a.? ~ b optimization, if we want consistency, it is better to just remove Rep0 entirely and represent it in the same way so we don't have any chance of breaking the symmetry there.