Use new exception for handling absent policy

For: QubesOS/qubes-issues#10746
Requires: QubesOS/qubes-core-qrexec#223

Author: ben-grande

qubes_config/global_config/policy_manager.py

@@ -21,6 +21,7 @@
 import subprocess
 from typing import Optional, List, Tuple
 
+from qrexec.policy.admin import PolicyAdminFileNotFoundException
 from qrexec.policy.admin_client import PolicyClient
 from qrexec.policy.parser import StringPolicy, Rule
 from qubes_config.widgets.utils import compare_rule_lists
@@ -83,7 +84,7 @@ def get_rules_from_filename(
         for the file."""
         try:
             rules_text, token = self.policy_client.policy_get(filename)
-        except subprocess.CalledProcessError:
+        except PolicyAdminFileNotFoundException:
             if not default_policy:
                 return [], None
             rules_text, token = default_policy, None