all: implement policy failover rcodes

While at it, ensure that config is validated, and fixing a bug related
to reuse ctx between multiple upstreams resolving.

internal/dnsrcode/rcode_test.go

@@ -0,0 +1,29 @@
+package dnsrcode
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFromString(t *testing.T) {
+	tests := []struct {
+		name          string
+		rcode         string
+		expectedRcode int
+	}{
+		{"valid", "NoError", 0},
+		{"upper", "NOERROR", 0},
+		{"lower", "noerror", 0},
+		{"mix", "nOeRrOr", 0},
+		{"invalid", "foo", -1},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			assert.Equal(t, tc.expectedRcode, FromString(tc.rcode))
+		})
+	}
+}