Return non-zero exit code for invalid CLI flags

[autrilla]

Fixes #470

[codecov-io]

Codecov Report

Merging #495 into develop will increase coverage by 0.18%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #495      +/-   ##
===========================================
+ Coverage    36.35%   36.53%   +0.18%     
===========================================
  Files           20       20              
  Lines         2773     2773              
===========================================
+ Hits          1008     1013       +5     
+ Misses        1669     1665       -4     
+ Partials        96       95       -1

Impacted Files	Coverage Δ
stores/flatten.go	91.52% <0%> (+4.23%)	⬆️

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