[BEAM-14346] Fix incorrect error case index in ret2()

[jrmccluskey]

ret2() had an incorrect index in the error check, leading to error cases getting skipped over improperly.

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Codecov Report

Merging #17425 (96ef8ab) into master (e4d2050) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #17425      +/-   ##
==========================================
+ Coverage   73.83%   73.85%   +0.02%     
==========================================
  Files         686      686              
  Lines       90143    90143              
==========================================
+ Hits        66555    66574      +19     
+ Misses      22406    22390      -16     
+ Partials     1182     1179       -3     

Flag	Coverage Δ
go	49.62% <100.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
sdks/go/pkg/beam/core/runtime/exec/fn_arity.go	7.69% <ø> (+1.21%)	⬆️
sdks/go/pkg/beam/core/runtime/exec/fn.go	60.78% <100.00%> (+5.22%)	⬆️

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[lostluck]

LGTM Thanks!