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.

cmd/stvanity/main.go

@@ -14,6 +14,7 @@ import (
 	"crypto/x509"
 	"crypto/x509/pkix"
 	"encoding/pem"
+	"errors"
 	"flag"
 	"fmt"
 	"math/big"
@@ -191,7 +192,7 @@ func pemBlockForKey(priv interface{}) (*pem.Block, error) {
 		}
 		return &pem.Block{Type: "EC PRIVATE KEY", Bytes: b}, nil
 	default:
-		return nil, fmt.Errorf("unknown key type")
+		return nil, errors.New("unknown key type")
 	}
 }