cli: print and exit instead of panic on error, improve benchmark read failure error

[bobheadxi]

Closes #51

[codecov]

Codecov Report

Merging #52 (8466bda) into master (b37e8fd) will decrease coverage by 0.43%.
The diff coverage is 11.63%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   31.30%   30.87%   -0.42%     
==========================================
  Files           8        8              
  Lines         441      460      +19     
==========================================
+ Hits          138      142       +4     
- Misses        285      302      +17     
+ Partials       18       16       -2     

Impacted Files	Coverage Δ
main.go	0.00% <0.00%> (ø)
bench/parser.go	69.12% <100.00%> (+5.89%)	⬆️

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