[Core] Prevent DB Failure from Breaking Enforcer

[lloyd-brown]

Before this PR while we are lazy initializing the PermissionService if we fail after having set the global enforcer instance then our enforcer is still none but we change the enforcer instance that we use on every subsequent call. That leads to future calls trying to use a None object to load policies which breaks requests.

This PR moves our change of the global enforcer instance until after we've successfully created an enforcer so that a DB failure will not break the permission service. I added a unit test to catch this in the future.

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)

[lloyd-brown]

/smoke-test