Support runtime errors.

[lambdafu]

See #44.

[codecov]

Codecov Report

Merging #45 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
- Coverage     100%   99.91%   -0.09%     
==========================================
  Files           9        9              
  Lines        1171     1179       +8     
  Branches      240      241       +1     
==========================================
+ Hits         1171     1178       +7     
- Misses          0        1       +1

Impacted Files	Coverage Δ
include/CLI/App.hpp	99.84% <100%> (-0.16%)	⬇️
include/CLI/Error.hpp	100% <100%> (ø)	⬆️

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[henryiii]

Thanks!