Allow to set custom error handler for dispatcher

[ark0f]

And extract default error handler in command dispatcher

[codecov-io]

Codecov Report

Merging #59 into master will decrease coverage by <.01%. The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
- Coverage   50.53%   50.53%   -0.01%     
==========================================
  Files         148      148              
  Lines        9595     9603       +8     
  Branches     2493     2491       -2     
==========================================
+ Hits         4849     4853       +4     
- Misses       2262     2265       +3     
- Partials     2484     2485       +1

Impacted Files	Coverage Δ
carapax/src/dispatcher.rs	52.23% <26.66%> (-4.91%)	:arrow_down:
carapax/src/command.rs	44.44% <61.53%> (+3.09%)	:arrow_up:

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[rossnomann]

Merged