fix(appsettings,vrtManager,storages): error handlers that hid the real failure

Reviewing the error handlers outside instances/ turned up three that masked or
swallowed the failure they caught:

- appsettings/settings.get_settings() runs on every request (middleware) and
  loads app_settings from the database. Its bare `except: pass` left `entries`
  unbound on a database error, so the next line raised UnboundLocalError
  instead; it also caught KeyboardInterrupt/SystemExit. It now catches
  DatabaseError and returns, keeping the already-loaded values.
- vrtManager.network.refresh_dhcp_leases raised a formatted string
  (`raise f"..."`), which Python rejects with TypeError, hiding the real
  DHCPLeases failure. It now raises util.OperationError, which the API/HTML
  error handling already renders as a libvirt error, and clears self.leases.
- storages.views.get_volumes caught libvirtError and answered 200 with an empty
  body, so the caller could not tell a failed pool read from an empty pool. It
  now returns 400 with the error.

The other broad handlers reviewed (appsettings/nwfilters/virtsecrets views,
storages/upload cleanup and recovery paths, vrtManager display getters) report
the error or are best-effort and were left as they are.

Tests: get_settings() survives a database error during the query and during
iteration without raising and still loads a real value; refresh_dhcp_leases
raises OperationError (keeping the cause) not TypeError; get_volumes returns 400
on a listing failure and the sorted volumes on success. The tests fail on the
previous code.

vrtManager/network.py

@@ -397,7 +397,7 @@ class wvmNetwork(wvmConnect):
             self.leases = self.net.DHCPLeases()
         except Exception as e:
             self.leases = []
-            raise f"Error getting {self} DHCP leases: {e}" from e
+            raise util.OperationError(f"Error getting {self} DHCP leases: {e}") from e
 
     def get_dhcp_leases(self):
         if self.leases is None:

vrtManager/test_dhcp_leases.py

@@ -0,0 +1,36 @@
+"""refresh_dhcp_leases must raise a real exception (handled as a libvirt error),
+not a bare string (which Python turns into TypeError, masking the cause)."""
+
+import unittest
+from unittest.mock import Mock
+
+from django.conf import settings
+
+if not settings.configured:
+    settings.configure(MAC_OUI="52:54:10")
+
+from vrtManager import util
+from vrtManager.network import wvmNetwork
+
+
+def network_with_leases(side_effect):
+    net = wvmNetwork.__new__(wvmNetwork)
+    net.leases = None
+    net.net = Mock()
+    net.net.DHCPLeases.side_effect = side_effect
+    return net
+
+
+class RefreshDhcpLeasesTestCase(unittest.TestCase):
+    def test_a_lease_error_raises_an_operation_error(self):
+        net = network_with_leases(Exception("no DHCP on this network"))
+        with self.assertRaises(util.OperationError):
+            net.refresh_dhcp_leases()
+
+    def test_the_error_keeps_the_cause_and_clears_leases(self):
+        cause = Exception("boom")
+        net = network_with_leases(cause)
+        with self.assertRaises(util.OperationError) as caught:
+            net.refresh_dhcp_leases()
+        self.assertIs(caught.exception.__cause__, cause)
+        self.assertEqual(net.leases, [])