OpenScreenTagList() and its variants return a pointer to a Screen
structure on the successful creation of a new screen and NULL on failure.
With V36, the call now supports extended error codes on failure.  The
error returns provide information on the type of failure, giving the
application a greater chance of recovery.  To get the extended error code,
you need to use the SA_ErrorCode tag; the code itself will be placed into
the LONG pointed to by the TagItem.ti_Data field.  Here are the codes:

OSERR_NOMONITOR
    The monitor needed to display the requested mode is not available. An
    example of this error would be opening a Productivity mode screen on
    a system without a VGA or multisync monitor.

OSERR_NOCHIPS
    Newer custom chips are required for this screen mode.  For instance,
    the ECS Denise is required for the productivity modes.

OSERR_NOMEM
    Could not allocate enough memory.

OSERR_NOCHIPMEM
    Could not allocate enough Chip memory.

OSERR_PUBNOTUNIQUE
    Could not create public screen--name already used.  The system
    requires that public screen names be unique.

OSERR_UNKNOWNMODE
    Display mode requested was not recognized.  The system does not
    understand the value specified with the SA_DisplayID tag.