operational error handling in poll_query_until()

[gsmolk]

useful for catching 'FATAL: the database system is starting up'

[codecov-io]

Codecov Report

Merging #50 into master will decrease coverage by 0.12%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   97.18%   97.06%   -0.13%     
==========================================
  Files          17       17              
  Lines        1564     1568       +4     
==========================================
+ Hits         1520     1522       +2     
- Misses         44       46       +2

Impacted Files	Coverage Δ
testgres/connection.py	95.16% <100%> (+0.07%)	⬆️
testgres/node.py	96.61% <33.33%> (-0.47%)	⬇️

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