add unit test for 503

[bill1600]

Change connection logic so that after an error that is not 403 or 3xx (redirect), we retry find_servers.
Also add unit test for this (code 503).

[codecov]

Codecov Report

Merging #247 into master will increase coverage by 0.07%.
The diff coverage is 97.82%.

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
+ Coverage   88.84%   88.91%   +0.07%     
==========================================
  Files          50       50              
  Lines        8615     8646      +31     
  Branches      673      673              
==========================================
+ Hits         7654     7688      +34     
+ Misses        316      314       -2     
+ Partials      645      644       -1

Impacted Files	Coverage Δ
tests/test_connection.c	99.68% <100%> (+0.01%)	⬆️
src/connection.c	76.72% <50%> (+0.64%)	⬆️
tests/test_service_alive.c	84% <0%> (+1.17%)	⬆️

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[shilpa24balaji]

@bill1600 Please update the CHANGE log

[shilpa24balaji]

Please update CHANGE log