Return errors in JSON converters

[alexvanin]

There is a proposal from @im-kulikov to return errors in JSON converters because these converters work with raw []byte and error nil check is more clear for go-developers in case of data marshaling.

[codecov]

Codecov Report

Merging #185 into master will increase coverage by 0.20%.
The diff coverage is 72.00%.

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
+ Coverage   40.96%   41.16%   +0.20%     
==========================================
  Files          74       74              
  Lines        8364     8349      -15     
==========================================
+ Hits         3426     3437      +11     
+ Misses       4466     4453      -13     
+ Partials      472      459      -13     

Impacted Files	Coverage Δ
v2/acl/json.go	66.66% <66.66%> (+28.20%)	⬆️
v2/container/json.go	80.00% <80.00%> (+41.53%)	⬆️
v2/netmap/json.go	80.00% <80.00%> (+41.53%)	⬆️

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[im-kulikov]

Thanks!