fix: move cssom to dependencies

[zamotany]

Fixes #124

Also, ensure we throw original error if it fails to enhance it.

[codecov]

Codecov Report

Merging #131 into master will decrease coverage by 0.23%.
The diff coverage is 80%.

@@            Coverage Diff            @@
##           master    #131      +/-   ##
=========================================
- Coverage   97.73%   97.5%   -0.24%     
=========================================
  Files          19      19              
  Lines         398     401       +3     
  Branches       74      74              
=========================================
+ Hits          389     391       +2     
- Misses          9      10       +1

Impacted Files	Coverage Δ
src/babel/lib/moduleSystem.js	98.55% <80%> (-1.45%)	⬇️

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