Fix panic when exit code does not have a cause

[HeavyWombat]

Fixes: #448

Use err.Error() of ExitCode interface since this has a nil check.

Move ExitCode into its own source file.

Fix TYPO in comment about error.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.38%. Comparing base (2fdda00) to head (b1ec0fc).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/cmd/exitcode.go	66.66%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #452   +/-   ##
=======================================
  Coverage   81.38%   81.38%           
=======================================
  Files          16       17    +1     
  Lines        2159     2159           
=======================================
  Hits         1757     1757           
  Misses        326      326           
  Partials       76       76           

Flag	Coverage Δ
unittests	81.38% <66.66%> (ø)

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