INT 15 - DESQview v2.20+ - SET ERROR HANDLING
        AX = DE15h
        BL = 00h post system error on all error conditions
             01h return carry flag set on calls to ADDTO, SUBFROM, and WRITE
                 messages sent to mailboxes which fail due to lack of system
                 or common memory
             02h (v2.26+) same as 01h, but return null pointer for GETMEM
                 calls which fail due to lack of system memory
Return: nothing