Undocumented error codes for custom transport

[Ryanf55]

It's not clear what the error codes are used in the custom transport.

size_t my_custom_transport_read(
        uxrCustomTransport* transport,
        uint8_t* buffer,
        size_t length,
        int timeout,
        uint8_t* errcode)
{
    ...
}

The error code is a pointer to a uint8_t. In ArduPilot, which is based on ChibiOS, we have written a custom transport adapter here, which passes error codes from LWIP to MicroXRCEDDS like so:
https://github.com/ArduPilot/ardupilot/blob/c54529aaf850390f7cc3d1583541e0055fc4c7ad/libraries/AP_DDS/AP_DDS_UDP.cpp#L65

The trouble is, what are the actual error codes used by MicroXRCEDDSGen?

I would much prefer if this was exposed as an enum so we could tell whether the codes were matched up, or at least documented what the supported codes are.

The result of this is a long-standing bug in ArduPilot - UDP transport doesn't work ChibiOS:
ArduPilot/ardupilot#25784

Should we expect a code 11 to result in MicroXRCEDDSAgent trying a read/write at a later time and not to drop the data?