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/upgrade/upgrade_supported.go

@@ -15,6 +15,7 @@ import (
 	"compress/gzip"
 	"crypto/tls"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -366,10 +367,10 @@ func archiveFileVisitor(dir string, tempFile *string, signature *[]byte, archive
 
 func verifyUpgrade(archiveName, tempName string, sig []byte) error {
 	if tempName == "" {
-		return fmt.Errorf("no upgrade found")
+		return errors.New("no upgrade found")
 	}
 	if sig == nil {
-		return fmt.Errorf("no signature found")
+		return errors.New("no signature found")
 	}
 
 	l.Debugf("checking signature\n%s", sig)