Return error when required flag is missing

[condemil]

Fixes #31

[codecov-io]

Codecov Report

Merging #33 into master will decrease coverage by 0.98%.
The diff coverage is 57.89%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   65.37%   64.39%   -0.99%     
==========================================
  Files           4        4              
  Lines         335      337       +2     
==========================================
- Hits          219      217       -2     
- Misses         82       87       +5     
+ Partials       34       33       -1

Impacted Files	Coverage Δ
cli/cli.go	82.71% <57.89%> (-4.63%)	⬇️

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[davrodpin]

Thanks for contributing to the project, @condemil!

Please rebase with master so I can approve it.

[condemil]

Rebased

[davrodpin]

Thanks!