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/protocol/hello.go

@@ -5,7 +5,6 @@ package protocol
 import (
 	"encoding/binary"
 	"errors"
-	"fmt"
 	"io"
 )
 
@@ -66,7 +65,7 @@ func readHello(c io.Reader) (HelloResult, error) {
 		}
 		msgSize := binary.BigEndian.Uint16(header[:2])
 		if msgSize > 32767 {
-			return HelloResult{}, fmt.Errorf("hello message too big")
+			return HelloResult{}, errors.New("hello message too big")
 		}
 		buf := make([]byte, msgSize)
 		if _, err := io.ReadFull(c, buf); err != nil {

lib/protocol/luhn.go

@@ -27,7 +27,7 @@ func (a luhnAlphabet) generate(s string) (rune, error) {
 	for i := range s {
 		codepoint := strings.IndexByte(string(a), s[i])
 		if codepoint == -1 {
-			return 0, fmt.Errorf("Digit %q not valid in alphabet %q", s[i], a)
+			return 0, fmt.Errorf("digit %q not valid in alphabet %q", s[i], a)
 		}
 		addend := factor * codepoint
 		if factor == 2 {