TUN-3767: Tolerate logging errors

This addresses a bug where logging would not be output when
cloudflared was run as a Windows Service.

That was happening because Windows Services have no stderr/out,
and the ConsoleWriter log was failing inside zerolog, which would
then not proceed to the next logger (the file logger).

We now overcome that by using our own multi writer that is resilient
to errors.

logger/create_test.go

@@ -0,0 +1,89 @@
+package logger
+
+import (
+	"github.com/pkg/errors"
+	"github.com/rs/zerolog"
+	"github.com/stretchr/testify/assert"
+	"io"
+	"testing"
+)
+
+var writeCalls int
+
+type mockedWriter struct {
+	wantErr bool
+}
+
+func (c mockedWriter) Write(p []byte) (int, error) {
+	writeCalls++
+
+	if c.wantErr {
+		return -1, errors.New("Expected error")
+	}
+
+	return len(p), nil
+}
+
+// Tests that a new writer is only used if it actually works.
+func TestResilientMultiWriter(t *testing.T) {
+	tests := []struct {
+		name    string
+		writers []io.Writer
+	}{
+		{
+			name:    "All valid writers",
+			writers: []io.Writer{
+				mockedWriter {
+					wantErr: false,
+				},
+				mockedWriter {
+					wantErr: false,
+				},
+			},
+		},
+		{
+			name:    "All invalid writers",
+			writers: []io.Writer{
+				mockedWriter {
+					wantErr: true,
+				},
+				mockedWriter {
+					wantErr: true,
+				},
+			},
+		},
+		{
+			name:    "First invalid writer",
+			writers: []io.Writer{
+				mockedWriter {
+					wantErr: true,
+				},
+				mockedWriter {
+					wantErr: false,
+				},
+			},
+		},
+		{
+			name:    "First valid writer",
+			writers: []io.Writer{
+				mockedWriter {
+					wantErr: false,
+				},
+				mockedWriter {
+					wantErr: true,
+				},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		writers := tt.writers
+		multiWriter := resilientMultiWriter{writers}
+
+		logger := zerolog.New(multiWriter).With().Timestamp().Logger().Level(zerolog.InfoLevel)
+		logger.Info().Msg("Test msg")
+
+		assert.Equal(t, len(writers), writeCalls)
+		writeCalls = 0
+	}
+}