[Bug Fix] Fix ValidationException error in which deleted endpoint cannot be found

[atqy]

Issue #, if available:
Fixes #2340
Fixes #2339

Description of changes: There were two major changes:

1. Flipped delete_endpoint and delete_model commands so that when delete_model is called, it would not throw an error saying that the already deleted endpoint is missing
2. Added a try and except clause to invoke_endpoint_forever function so that it will handle the error in the case that the background thread is still running and the endpoint is deleted at the end of the notebook.

Testing done: Ran through both notebooks manually end-to-end, no errors were thrown.

Merge Checklist

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• I have read the CONTRIBUTING doc and adhered to the example notebook best practices
• I have updated any necessary documentation, including READMEs
• I have tested my notebook(s) and ensured it runs end-to-end
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