#635 feat: add user defined exception for use for general pod spawn errors

[zsinnema]

Closes #635

Implemented solution: added a "generic" Pod Spawn Exception to CH jupyterhub.py that is catched and raised again instead of the generic catch all log and continue exception

How to test this PR:

• import PodSpawnException from harness_jupyter jupyterhub
• raise this exception

Sanity checks:

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• From the issue and the current PR it is explicitly stated how to test the current change
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Breaking changes (select one):

• The present changes do not change the preexisting api in any way
• This PR and the issue are tagged as a breaking-change

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• This PR and the issue are tagged as alert:deployment

Test coverage (select one):

• Tests for the relevant cases are included in this pr
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Documentation (select one):

• The documentation has been updated to match the current changes
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Nice to have (if relevant):

• Screenshots of the changes
• Explanatory video/animated gif

[filippomc]

lgtm