fix(netxlite/dns): more stricly mirror stdlib error strings (#513)

This diff attempts to modify the errors reported by our custom
resolver by matching more strings from the stdlib.

Part of https://github.com/ooni/probe/issues/1733 and diff has been
extracted from https://github.com/ooni/probe-cli/pull/506.

internal/netxlite/dnsx/dnsoverhttps_test.go

@@ -175,3 +175,18 @@ func TestDNSOverHTTPSHostOverride(t *testing.T) {
 		t.Fatal("did not see correct host override")
 	}
 }
+
+func TestDNSOverHTTPSCloseIdleConnections(t *testing.T) {
+	var called bool
+	doh := &DNSOverHTTPS{
+		Client: &mocks.HTTPClient{
+			MockCloseIdleConnections: func() {
+				called = true
+			},
+		},
+	}
+	doh.CloseIdleConnections()
+	if !called {
+		t.Fatal("not called")
+	}
+}