Is it an error if can't resolve uri-reference?

Is it an error if, when processing an include or import element, a 
stylesheet processor can't resolve the href uri-reference?
The 1.0 spec doesn't say that it is. Or did I miss it?

I see that Xalan reports it as an error. Saxon 7.8 probably does too, 
though what I see is it doesn't catch the FileNotFound exception.
Bob Foster