Check for serverapp for reraise flag

[vidartf]

The secondary serverextension apps (enable/disable/list) should probably always complete even if an extension raises an error, so in those cases, default to just warning about exceptions, but still complete.

Fixes #676

Note: Not tested locally, just edited via GH interface. I'd prefer to add a regression test as well if I have time.

[codecov-commenter]

Codecov Report

Merging #887 (f090a51) into main (6baed5e) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #887      +/-   ##
==========================================
- Coverage   72.15%   72.12%   -0.03%     
==========================================
  Files          65       65              
  Lines        8044     8044              
  Branches     1343     1343              
==========================================
- Hits         5804     5802       -2     
- Misses       1835     1836       +1     
- Partials      405      406       +1     

Impacted Files	Coverage Δ
jupyter_server/extension/manager.py	92.68% <100.00%> (ø)
jupyter_server/services/kernels/handlers.py	58.64% <0.00%> (-0.43%)	⬇️

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[Zsailer]

Thanks, @vidartf!