INT 2F U - DOS 3+ CRITICAL ERROR HANDLER - EXPAND ERROR INTO STRING
        AH = 05h
---DOS 3.x---
        AL = extended error code (not zero)
---DOS 4+ ---
        AL = error type
            01h DOS extended error code
            02h parameter error
        BX = error code
Return: CF clear if successful
            ES:DI -> ASCIZ error message (read-only)
            AL = ???
        CF set if error code can't be converted to string
Notes:  called at start of COMMAND.COM's default critical error handler if
          installed by a user program, allowing partial or complete overriding
          of the default error message
        subfunction 02h called by many DOS 4 external programs
SeeAlso: INT 24