Fix connector convert OSError to ClientConnectorError

[hellysmile]

What do these changes do?

Fix for #2423

Are there changes in behavior for the user?

When dns request fails OSError converted ClientConnectorError

#2423

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new news fragment into the changes folder
  • name it <issue_id>.<type> for example (588.bug)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[codecov-io]

Codecov Report

Merging #2430 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2430      +/-   ##
==========================================
+ Coverage   97.19%   97.19%   +<.01%     
==========================================
  Files          39       39              
  Lines        8152     8155       +3     
  Branches     1419     1419              
==========================================
+ Hits         7923     7926       +3     
  Misses         99       99              
  Partials      130      130

Impacted Files	Coverage Δ
aiohttp/connector.py	97.24% <100%> (+0.01%)	⬆️

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[hellysmile]

target branch is 2.3 #2442

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new