GuildBan method and UnknownBan error code addition (#848)

* UnknownBan error code addition

* GuildBan method implementation

* Gofmt fix

Gofmt fix

restapi.go

@@ -693,6 +693,19 @@ func (s *Session) GuildBanCreate(guildID, userID string, days int) (err error) {
 	return s.GuildBanCreateWithReason(guildID, userID, "", days)
 }
 
+// GuildBan finds ban by given guild and user id and returns GuildBan structure
+func (s *Session) GuildBan(guildID, userID string) (st *GuildBan, err error) {
+
+	body, err := s.RequestWithBucketID("GET", EndpointGuildBan(guildID, userID), nil, EndpointGuildBan(guildID, userID))
+	if err != nil {
+		return
+	}
+
+	err = unmarshal(body, &st)
+
+	return
+}
+
 // GuildBanCreateWithReason bans the given user from the given guild also providing a reaso.
 // guildID   : The ID of a Guild.
 // userID    : The ID of a User

structs.go

@@ -1253,6 +1253,7 @@ const (
 	ErrCodeUnknownUser        = 10013
 	ErrCodeUnknownEmoji       = 10014
 	ErrCodeUnknownWebhook     = 10015
+	ErrCodeUnknownBan         = 10026
 
 	ErrCodeBotsCannotUseEndpoint  = 20001
 	ErrCodeOnlyBotsCanUseEndpoint = 20002