docs: def get_supported_boot_mode(self):
          """Get the system supported boot modes.
  
          :return: any one of the following proliantutils.ilo.constants:
  
              SUPPORTED_BOOT_MODE_LEGACY_BIOS_ONLY,
              SUPPORTED_BOOT_MODE_UEFI_ONLY,
              SUPPORTED_BOOT_MODE_LEGACY_BIOS_AND_UEFI
          :raises: IloError, if account not found or on an error from iLO.
          """
          sushy_system = self._get_sushy_system(PROLIANT_SYSTEM_ID)
          try:
              return SUPPORTED_BOOT_MODE_MAP.get(
                  sushy_system.supported_boot_mode)
          except sushy.exceptions.SushyError as e:
              msg = (self._('The Redfish controller failed to get the '
                            'supported boot modes. Error: %s') % e)
              LOG.debug(msg)
              raise exception.IloError(msg)