This routine performs a SCSI Request Sense command, or equivalent
    if some other I/O architecture is supported. It is used by all
    MRD API routines to determine the cause of a command failure.

    The robot_info is the address of a robot_info_t structure that
    has been opened by mrd_startup(3mrd). If the medium changer isn't
    opened, the Request Sense command will fail with the operating
    system error for trying to use an unopened device.

    The dev_status_t structure includes the code, os_status, and SCSI
    error fields. The following describes how to decode errors with
    the dev_status_t structure.

    SCSI Errors

    SCSI errors are indicated when the value of the valid field of
    the SCSI error is not equal to 0. The key, asc, and ascq fields
    provide additional information to help determine the cause of the
    error.

    The code usually maps the Additional Sense Code and Additional
    Sense Code Qualifier (ASC/ASCQ) values to an MRD error. The asc
    and ascq values are copied from the request sense data returned
    by the target.

    The Additional Sense Code (asc) indicates further information
    related to the error or exception condition reported in the sense
    key field. The Additional Sense Code Qualifier (ascq) indicates
    detailed information related to the additional sense code. For
    more information, consult the SCSI-2 Specification.

    Operating System Errors

    Operating system errors are indicated when the value of the valid
    field of the SCSI error is equal to 0 and the value of the os_
    status field is not equal to 0. This result is most likely caused
    by an operating system error, and probably has a mapped error in
    MRD.

    MRD Errors

    MRD errors are indicated when the value of the os_status field is
    0, and the value of the valid field of the SCSI error is 0. This
    result is most likely caused when MRD encounters its own failure.

    In typical usage by MRD, the os_status argument will be an
    operating system specific code. However, The SCSI-2 specification
    allows Request Sense to be used at any time to obtain status
    information about a device. To support this feature, the MRD
    implementation of mrd_reqeust_sense(3mrd) can be called with the
    code MRD_CHECK_SENSE to force a Request Sense command.