Do not log connection error if the kernel is already shutdown

[martinRenou]

I had the following error locally:

[Voila] Kernel shutdown: c7f832d5-6b46-4f77-9f98-f0b9ca6e7ac8
[Voila] WARNING | Timeout waiting for kernel_info reply from c7f832d5-6b46-4f77-9f98-f0b9ca6e7ac8
[Voila] ERROR | Error opening stream: HTTP 404: Not Found (Kernel does not exist: c7f832d5-6b46-4f77-9f98-f0b9ca6e7ac8)

This was because I closed the Voila application before the client could
finish connecting to the kernel.

With this PR, it should shutdown the kernel properly and be silent after
not being able to connect.

[codecov-commenter]

Codecov Report

Merging #584 (e2235d7) into master (67cdb6b) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #584   +/-   ##
=======================================
  Coverage   77.39%   77.39%           
=======================================
  Files         110      110           
  Lines       10208    10212    +4     
  Branches     1104     1104           
=======================================
+ Hits         7900     7904    +4     
+ Misses       1918     1915    -3     
- Partials      390      393    +3     

Impacted Files	Coverage Δ
jupyter_server/services/kernels/handlers.py	60.92% <0.00%> (-0.64%)	⬇️
jupyter_server/tests/services/kernels/test_api.py	96.93% <0.00%> (-2.05%)	⬇️
jupyter_server/services/kernels/kernelmanager.py	81.84% <0.00%> (+2.05%)	⬆️

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[blink1073]

Thanks!