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

### 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_