code: public static TransportErrorCode fromHttp(int code) {
          TransportErrorCode builtIn = HTTP_ERROR_CODE_MAP.get(code);
          if (builtIn == null) {
              if (code > 599 || code < 100) {
                  throw new IllegalArgumentException("Invalid http status code: " + code);
              } else if (code < 400) {
                  throw new IllegalArgumentException("Invalid http error code: " + code);
              } else {
                  return new TransportErrorCode(code, 4000 + code, "Unknown error code");
              }
          } else {
              return builtIn;
          }
      }
docs: Get a transport error code from the given HTTP error code.
  
  @param code The HTTP error code, must be between 400 and 599 inclusive.
  @return The transport error code.
  @throws IllegalArgumentException if the HTTP code was not between 400 and 599.