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/relay/protocol/protocol.go

@@ -3,7 +3,7 @@
 package protocol
 
 import (
-	"fmt"
+	"errors"
 	"io"
 )
 
@@ -53,7 +53,7 @@ func WriteMessage(w io.Writer, message interface{}) error {
 		payload, err = msg.MarshalXDR()
 		header.messageType = messageTypeRelayFull
 	default:
-		err = fmt.Errorf("Unknown message type")
+		err = errors.New("unknown message type")
 	}
 
 	if err != nil {
@@ -84,7 +84,7 @@ func ReadMessage(r io.Reader) (interface{}, error) {
 	}
 
 	if header.magic != magic {
-		return nil, fmt.Errorf("magic mismatch")
+		return nil, errors.New("magic mismatch")
 	}
 
 	buf = make([]byte, int(header.messageLength))
@@ -127,5 +127,5 @@ func ReadMessage(r io.Reader) (interface{}, error) {
 		return msg, err
 	}
 
-	return nil, fmt.Errorf("Unknown message type")
+	return nil, errors.New("unknown message type")
 }