Add ability to abort from write_flash in openlcb/Bootloader.hxx

[atanisoft]

When running the bootloader to update firmware it can be useful to abort the process as soon as there is an error in write_flash. Today there is no way to handle this other than rebooting the node entirely.

An example usecase is with the the ESP32 firmware, the first segment (~256b or so) contains firmware identification data (target chip id, size, etc). If the firmware is not for the currently running SoC it doesn't make sense to continue reading the rest of the firmware over CAN and instead abort the request immediately with an error code like ERROR_FIRMWARE_INCOMPATIBLE.

Other similar use cases could be considered and would allow flexibility on aborting earlier in the stream.