Add more tests on error handling and responses for invalid inputs in Policy APIs

[HonahX]

Is your feature request related to a problem? Please describe.

Currently, we have tests verifying correct outputs for valid inputs but lacks coverage for invalid inputs to ensure the correct exception/error message is thrown. It will be good to add those.

example:

1. create policy under a non-existing namespace
2. attach/detach policy to non-existing target (catalog/namesopace/table)
3. getApplicablePolicies on non-existing catalog/namesopace/table

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response

[williamhyun]

Hi @HonahX, I would love to work on this. Could you please assign it to me?
Thank you!