handle KeyError when get session

[ccw630]

Port of jupyter/notebook#6245

#110

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[codecov-commenter]

Codecov Report

Merging #641 (f27255b) into main (606487a) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #641      +/-   ##
==========================================
- Coverage   77.78%   77.69%   -0.09%     
==========================================
  Files         110      110              
  Lines       10462    10465       +3     
  Branches     1419     1419              
==========================================
- Hits         8138     8131       -7     
- Misses       1923     1934      +11     
+ Partials      401      400       -1     

Impacted Files	Coverage Δ
jupyter_server/services/sessions/sessionmanager.py	87.50% <100.00%> (+0.26%)	⬆️
...ter_server/tests/services/sessions/test_manager.py	100.00% <100.00%> (ø)
jupyter_server/services/kernels/kernelmanager.py	81.26% <0.00%> (-1.91%)	⬇️
jupyter_server/services/kernels/handlers.py	61.25% <0.00%> (-1.46%)	⬇️
jupyter_server/tests/services/kernels/test_api.py	97.60% <0.00%> (+1.59%)	⬆️

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[blink1073]

Thank you! The CI failures are unrelated.

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