runtime, internal/poll, net: report event scanning error on read event
This change makes it possible the runtime-integrated network poller and
APIs in the package internal/poll to report an event scanning error on a
read event.
The latest Go releases open up the way of the manipulation of the poller
for users. On the other hand, it starts misleading users into believing
that the poller accepts any user-configured file or socket perfectly
because of not reporting any error on event scanning, as mentioned in
issue 30426. The initial implementation of the poller was designed for
just well-configured, validated sockets produced by the package net.
However, the assumption is now obsolete.