I think the new behavior should be controlled by a constructor flag, maybe
named "defer_errors". I don't like the idea of adding the flag to read(),
since that makes us diverge from the standard file interface. Making a
distinction between size<0 and size=None seems confusing and error-prone,
not to mention that we (again) would have read() work differently from most
other file classes.

I'd prefer it if the new behavior is not enabled by default for size>=0,
even if this wouldn't break well-behaved code. Having a flag that only
controls the size<0 case is inelegant, and I don't think we should change
the default behavior unless there is a clear benefit to doing so.