hande ErrUnexpectedEndOfDemo and exit with code 3

[markus-wa]

see #46

[codecov]

Codecov Report

Merging #47 into master will decrease coverage by 0.85%.
The diff coverage is 28.57%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   93.20%   92.35%   -0.86%     
==========================================
  Files           6        6              
  Lines         633      641       +8     
==========================================
+ Hits          590      592       +2     
- Misses         23       28       +5     
- Partials       20       21       +1     

Impacted Files	Coverage Δ
csminify.go	90.17% <25.00%> (-2.49%)	⬇️
cmd/csminify/main.go	82.08% <33.33%> (-3.40%)	⬇️

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