[3.8] Custom error message for Unix connect errors (#4985)

[asvetlov]

Co-authored-by: Sviatoslav Sydorenko [email protected]

Co-authored-by: Andrew Svetlov [email protected]

[codecov]

Codecov Report

Merging #5264 (99663ed) into 3.8 (c7988ad) will decrease coverage by 0.02%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##              3.8    #5264      +/-   ##
==========================================
- Coverage   97.53%   97.51%   -0.03%     
==========================================
  Files          44       44              
  Lines        8844     8853       +9     
  Branches     1426     1426              
==========================================
+ Hits         8626     8633       +7     
- Misses        102      104       +2     
  Partials      116      116              

Flag	Coverage Δ
unit	97.33% <80.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
aiohttp/client_exceptions.py	98.50% <77.77%> (-1.50%)	⬇️
aiohttp/connector.py	96.69% <100.00%> (ø)

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