Update nestif API to provide just pure message

[nakabonne]

Re-defined the error format at the main package.

[nakabonne]

Codecov Report

Merging #5 into master will decrease coverage by 0.08%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
- Coverage   79.29%   79.21%   -0.09%     
==========================================
  Files           3        3              
  Lines         256      255       -1     
==========================================
- Hits          203      202       -1     
  Misses         40       40              
  Partials       13       13

Impacted Files	Coverage Δ
nestif.go	96.92% <100%> (-0.14%)	⬇️
cmd/nestif/main.go	68.66% <75%> (+0.42%)	⬆️

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