Enhancement: Throw exception when environment variable could not be unset

[localheinz]

This PR

• throws an exception when a system environment variable could not be unset

[codecov]

Codecov Report

Merging #15 into master will decrease coverage by 0.78%.
The diff coverage is 71.42%.

@@             Coverage Diff              @@
##             master      #15      +/-   ##
============================================
- Coverage     99.14%   98.36%   -0.79%     
- Complexity       64       66       +2     
============================================
  Files             9       10       +1     
  Lines           117      122       +5     
============================================
+ Hits            116      120       +4     
- Misses            1        2       +1

Impacted Files	Coverage Δ	Complexity Δ
src/Exception/CouldNotUnset.php	100% <100%> (ø)	1 <1> (?)
src/SystemVariables.php	90.9% <50%> (-4.1%)	14 <0> (+1)

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