Handle json parsing errors in rpc check

[badrogger]

Changes:

• Improved error handling of failed helathcheck rpc request to skaled.

Testing

• Added new unit tests.
• Retested basic functions on the internal network.

Perfomance

• No perfomance related changes were introduced.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.55%. Comparing base (aa85bd9) to head (e547f09).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1086      +/-   ##
===========================================
+ Coverage    81.52%   81.55%   +0.02%     
===========================================
  Files          103      103              
  Lines         6041     6049       +8     
===========================================
+ Hits          4925     4933       +8     
  Misses        1116     1116              

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