Handle accountant errors

[vkuznecovas]

Until now, all errors from accountant were treated equally. They are now classified and acted upon accordingly.

[codecov-io]

Codecov Report

Merging #1618 into master will increase coverage by 0.18%.
The diff coverage is 59.45%.

@@            Coverage Diff             @@
##           master    #1618      +/-   ##
==========================================
+ Coverage   48.43%   48.62%   +0.18%     
==========================================
  Files         291      291              
  Lines       11648    11693      +45     
==========================================
+ Hits         5642     5686      +44     
+ Misses       5594     5591       -3     
- Partials      412      416       +4

Impacted Files	Coverage Δ
session/pingpong/factory.go	0% <0%> (ø)	⬆️
session/pingpong/accountant_promise_settler.go	50.24% <0%> (-2.31%)	⬇️
session/pingpong/invoice_tracker.go	64.75% <65.78%> (+9.28%)	⬆️
session/pingpong/accountant_caller.go	72.91% <73.07%> (-4.87%)	⬇️
services/wireguard/connection/connection.go	64.74% <0%> (-2.16%)	⬇️
core/connection/manager.go	82.74% <0%> (-0.64%)	⬇️
tequilapi/sse/sse.go	74.44% <0%> (+3.33%)	⬆️

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