all: Modernize error wrapping (#8491)

This replaces old style errors.Wrap with modern fmt.Errorf and removes
the (direct) dependency on github.com/pkg/errors. A couple of cases are
adjusted by hand as previously errors.Wrap(nil, ...) would return nil,
which is not what fmt.Errorf does.

lib/model/util.go

@@ -7,13 +7,13 @@
 package model
 
 import (
+	"errors"
 	"fmt"
 	"path/filepath"
 	"strings"
 	"sync"
 	"time"
 
-	"github.com/pkg/errors"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/ur"