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/sharedpullerstate.go

@@ -8,11 +8,10 @@ package model
 
 import (
 	"encoding/binary"
+	"fmt"
 	"io"
 	"time"
 
-	"github.com/pkg/errors"
-
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/protocol"
@@ -177,12 +176,12 @@ func (s *sharedPullerState) tempFileInWritableDir(_ string) error {
 		// what the umask dictates.
 
 		if err := s.fs.Chmod(s.tempName, mode); err != nil {
-			return errors.Wrap(err, "setting perms on temp file")
+			return fmt.Errorf("setting perms on temp file: %w", err)
 		}
 	}
 	fd, err := s.fs.OpenFile(s.tempName, flags, mode)
 	if err != nil {
-		return errors.Wrap(err, "opening temp file")
+		return fmt.Errorf("opening temp file: %w", err)
 	}
 
 	// Hide the temporary file