question regarding scanw errors

[Ruslan Nabioullin]

Hello, the man page of scanw states "vwscanw  returns  ERR on 
failure..."; because scanw is an IO function, obviously failure implies 
"system" errors.  However, based on superficial testing, scanw returns 
ERR in case of the input, before the newline, being "blank" (empty or 
consisting of merely whitespace), instead of the intuitively-expected 0.

My question is, how does one distinguish whether a "system" error 
occurred or the input was "blank", upon return of ERR?

Thanks in advance,
Ruslan