fix: add clearer error message for post-subst unmarshal issues

[ecrupper]

The actual error reported by the YAML package is rarely useful. This will help users zero in on the issue

[wass3rw3rk]

i like

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 58.23%. Comparing base (28593d8) to head (b82267e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
compiler/native/substitute.go	0.00%	1 Missing ⚠️

❌ Your project status has failed because the head coverage (58.23%) is below the target coverage (90.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1415   +/-   ##
=======================================
  Coverage   58.23%   58.23%           
=======================================
  Files         660      660           
  Lines       25292    25292           
=======================================
  Hits        14729    14729           
  Misses       9879     9879           
  Partials      684      684           

Files with missing lines	Coverage Δ
compiler/native/substitute.go	69.23% <0.00%> (ø)

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