test: coverage for invalid use cases

[nfischer]

No change to logic. This adds test coverage for more missing test cases. This covers the invalid cases, like invoking 'shjs' without a script name argument.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e2dded8) to head (4b7ec22).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##             main       #25       +/-   ##
============================================
+ Coverage   77.27%   100.00%   +22.72%     
============================================
  Files           1         1               
  Lines          22        22               
============================================
+ Hits           17        22        +5     
+ Misses          5         0        -5     

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