ipn: don't log if legacy prefs don't exist (the normal case these days)

ipn/local.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"os"
 	"strings"
 	"sync"
 	"time"
@@ -669,7 +670,9 @@ func (b *LocalBackend) loadStateLocked(key StateKey, prefs *Prefs, legacyPath st
 			if legacyPath != "" {
 				b.prefs, err = LoadPrefs(legacyPath)
 				if err != nil {
-					b.logf("Failed to load legacy prefs: %v", err)
+					if !os.IsNotExist(err) {
+						b.logf("Failed to load legacy prefs: %v", err)
+					}
 					b.prefs = NewPrefs()
 				} else {
 					b.logf("Imported state from relaynode for %q", key)

ipn/prefs.go

@@ -228,11 +228,11 @@ func (p *Prefs) Clone() *Prefs {
 func LoadPrefs(filename string) (*Prefs, error) {
 	data, err := ioutil.ReadFile(filename)
 	if err != nil {
-		return nil, fmt.Errorf("loading prefs from %q: %v", filename, err)
+		return nil, fmt.Errorf("LoadPrefs open: %w", err) // err includes path
 	}
 	p, err := PrefsFromBytes(data, false)
 	if err != nil {
-		return nil, fmt.Errorf("decoding prefs in %q: %v", filename, err)
+		return nil, fmt.Errorf("LoadPrefs(%q) decode: %w", filename, err)
 	}
 	return p, nil
 }