fix: return invalid_grant error for invalid or expired auth codes

Signed-off-by: m.nabokikh <maksim.nabokikh@flant.com>

server/handlers.go

@@ -811,7 +811,7 @@ func (s *Server) handleAuthCode(w http.ResponseWriter, r *http.Request, client s
 			s.logger.Errorf("failed to get auth code: %v", err)
 			s.tokenErrHelper(w, errServerError, "", http.StatusInternalServerError)
 		} else {
-			s.tokenErrHelper(w, errInvalidRequest, "Invalid or expired code parameter.", http.StatusBadRequest)
+			s.tokenErrHelper(w, errInvalidGrant, "Invalid or expired code parameter.", http.StatusBadRequest)
 		}
 		return
 	}