json: fix implementation of json Unmarshalers.

* AuthenticationMD5Password was wrong and is not needed
* Bind was wrong
* ErrorResponse is not needed
* Minor improvements for reliability

authentication_md5_password.go

@@ -2,7 +2,6 @@ package pgproto3
 
 import (
 	"encoding/binary"
-	"encoding/json"
 	"errors"
 
 	"github.com/jackc/pgio"
@@ -42,24 +41,3 @@ func (src *AuthenticationMD5Password) Encode(dst []byte) []byte {
 	dst = append(dst, src.Salt[:]...)
 	return dst
 }
-
-// UnmarshalJSON implements encoding/json.Unmarshaler.
-func (dst *AuthenticationMD5Password) UnmarshalJSON(data []byte) error {
-	// Ignore null, like in the main JSON package.
-	if string(data) == "null" {
-		return nil
-	}
-
-	var msg struct {
-		Salt string
-	}
-	if err := json.Unmarshal(data, &msg); err != nil {
-		return err
-	}
-	if len(msg.Salt) != 4 {
-		return errors.New("invalid salt size")
-	}
-
-	copy(dst.Salt[:], []byte(msg.Salt)[:4])
-	return nil
-}