all: Tweak error creation (#6391)

- In the few places where we wrap errors, use the new Go 1.13 "%w"
  construction instead of %s or %v.

- Where we create errors with constant strings, consistently use
  errors.New and not fmt.Errorf.

- Remove capitalization from errors in the few places where we had that.

lib/versioner/util.go

@@ -7,7 +7,6 @@
 package versioner
 
 import (
-	"fmt"
 	"path/filepath"
 	"regexp"
 	"sort"
@@ -20,9 +19,11 @@ import (
 	"github.com/syncthing/syncthing/lib/util"
 )
 
-var errDirectory = fmt.Errorf("cannot restore on top of a directory")
-var errNotFound = fmt.Errorf("version not found")
-var errFileAlreadyExists = fmt.Errorf("file already exists")
+var (
+	errDirectory         = errors.New("cannot restore on top of a directory")
+	errNotFound          = errors.New("version not found")
+	errFileAlreadyExists = errors.New("file already exists")
+)
 
 // TagFilename inserts ~tag just before the extension of the filename.
 func TagFilename(name, tag string) string {

lib/versioner/versioner.go

@@ -9,6 +9,7 @@
 package versioner
 
 import (
+	"errors"
 	"fmt"
 	"time"
 
@@ -32,7 +33,7 @@ type factory func(filesystem fs.Filesystem, params map[string]string) Versioner
 
 var factories = make(map[string]factory)
 
-var ErrRestorationNotSupported = fmt.Errorf("version restoration not supported with the current versioner")
+var ErrRestorationNotSupported = errors.New("version restoration not supported with the current versioner")
 
 const (
 	TimeFormat = "20060102-150405"