feat: try to unwrap external errors during error wrapping

[morningvera]

closes #37

[morningvera]

Codecov Report

Merging #78 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #78      +/-   ##
==========================================
+ Coverage   98.85%   98.90%   +0.05%     
==========================================
  Files           3        3              
  Lines         261      274      +13     
==========================================
+ Hits          258      271      +13     
  Misses          1        1              
  Partials        2        2              

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[morningvera]

I still want to add an example showing this working with pkg/errors. Since their Unwrap method doesn't work as expected, I'll probably need to make some small change to make it work. No need to review until that's done.

[morningvera]

I also want to add test cases to stack_test.go and just discovered I need to fix some bugs. Seems like there's too much left to do so I'm closing it for now.