rpc: signal JSON marshalling errors through exception

[roman-khimov]

See neo-project/neo-modules#696. Unmarshaller is left unchanged to be
compatible.

[codecov]

Codecov Report

Merging #2461 (930c439) into master (3c0ee9c) will decrease coverage by 0.07%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master    #2461      +/-   ##
==========================================
- Coverage   85.10%   85.03%   -0.08%     
==========================================
  Files         291      291              
  Lines       36732    36736       +4     
==========================================
- Hits        31262    31239      -23     
- Misses       4155     4185      +30     
+ Partials     1315     1312       -3     

Impacted Files	Coverage Δ
pkg/services/oracle/network.go	75.86% <0.00%> (ø)
pkg/services/oracle/oracle.go	72.86% <ø> (-15.51%)	⬇️
pkg/rpc/response/result/invoke.go	81.74% <80.00%> (+0.59%)	⬆️
pkg/services/oracle/request.go	57.99% <0.00%> (-5.03%)	⬇️
pkg/network/server.go	74.11% <0.00%> (-0.10%)	⬇️
pkg/network/capability/capability.go	75.00% <0.00%> (+5.76%)	⬆️
pkg/network/message_string.go	77.41% <0.00%> (+6.45%)	⬆️

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