BUG: don't try to be clever with cumulative retcodes to avoid retcode overflow

[neutrinoceros]

Just learned that retcodes overflow past 255 as shown here

$ echo "exit 256" > /tmp/t.sh
$ bash /tmp/t.sh
$ echo $?
0

So let's not be clever with retcodes, keep them simple and small.

[codecov]

Codecov Report

Merging #97 (97bb9f4) into main (5ccaa65) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #97   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          637       637           
=========================================
  Hits           637       637           

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
inifix/__init__.py	100.00% <100.00%> (ø)
inifix/format.py	100.00% <100.00%> (ø)
inifix/validate.py	100.00% <100.00%> (ø)

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