errors: provide an error bot is not channel chat member (#659)

* errors: provide an error bot is not channel chat member

* errors: refactor

errors.go

@@ -133,6 +133,7 @@ var (
 	ErrKickedFromChannel    = NewError(403, "Forbidden: bot was kicked from the channel chat")
 	ErrNotStartedByUser     = NewError(403, "Forbidden: bot can't initiate conversation with a user")
 	ErrUserIsDeactivated    = NewError(403, "Forbidden: user is deactivated")
+	ErrNotChannelMember     = NewError(403, "Forbidden: bot is not a member of the channel chat")
 )
 
 // Err returns Error instance by given description.
@@ -250,6 +251,8 @@ func Err(s string) error {
 		return ErrChannelsTooMuch
 	case ErrChannelsTooMuchUser.ʔ():
 		return ErrChannelsTooMuchUser
+	case ErrNotChannelMember.ʔ():
+		return ErrNotChannelMember
 	default:
 		return nil
 	}