chore: do not use sentinel errors when unneeded

- main reason being it's a burden to always define sentinel errors at global scope, wrap them with `%w` instead of using a string directly
- only use sentinel errors when it has to be checked using `errors.Is`
- replace all usage of these sentinel errors in `fmt.Errorf` with direct strings that were in the sentinel error
- exclude the sentinel error definition requirement from .golangci.yml
- update unit tests to use ContainersError instead of ErrorIs so it stays as a "not a change detector test" without requiring a sentinel error

internal/configuration/settings/firewall_test.go

@@ -13,25 +13,21 @@ func Test_Firewall_validate(t *testing.T) {
 
 	testCases := map[string]struct {
 		firewall   Firewall
-		errWrapped error
 		errMessage string
 	}{
 		"empty": {
-			errWrapped: log.ErrLevelNotRecognized,
 			errMessage: "iptables settings: log level: level is not recognized: ",
 		},
 		"zero_vpn_input_port": {
 			firewall: Firewall{
 				VPNInputPorts: []uint16{0},
 			},
-			errWrapped: ErrFirewallZeroPort,
 			errMessage: "VPN input ports: cannot have a zero port",
 		},
 		"zero_input_port": {
 			firewall: Firewall{
 				InputPorts: []uint16{0},
 			},
-			errWrapped: ErrFirewallZeroPort,
 			errMessage: "input ports: cannot have a zero port",
 		},
 		"unspecified_outbound_subnet": {
@@ -40,7 +36,6 @@ func Test_Firewall_validate(t *testing.T) {
 					netip.MustParsePrefix("0.0.0.0/0"),
 				},
 			},
-			errWrapped: ErrFirewallPublicOutboundSubnet,
 			errMessage: "outbound subnet has an unspecified address: 0.0.0.0/0",
 		},
 		"public_outbound_subnet": {
@@ -70,9 +65,10 @@ func Test_Firewall_validate(t *testing.T) {
 
 			err := testCase.firewall.validate()
 
-			assert.ErrorIs(t, err, testCase.errWrapped)
-			if testCase.errWrapped != nil {
+			if testCase.errMessage != "" {
 				assert.EqualError(t, err, testCase.errMessage)
+			} else {
+				assert.NoError(t, err)
 			}
 		})
 	}