Don't report errors that are not caused by Accept in AcceptSocks.

This means that anything that is not a net.Error, or is a net.Error but
is not Temporary, should be considered to be a permanent error by the
caller. Sample code now shows the new error-checking convention.
Previously, we used the convention that a non-net.Error should be
considered temporary, because it could have been caused by a failed
SOCKS negotiation, for example. Now those errors are simply not returned
to the caller. See https://trac.torproject.org/projects/tor/ticket/14135.

In summary, previous behavior is this:
	net.Error, Temporary:     caller should try again
	net.Error, non-Temporary: caller should quit
	other errors:             caller should try again
It is now this:
	net.Error, Temporary:     caller should try again
	net.Error, non-Temporary: caller should quit
	other errors:             caller should quit
But now the "other errors" such as those caused by a bad SOCKS
negotiation will not be reported by AcceptSocks.

The practical effect of this change is almost nil; even if callers don't
update their error-checking code, the only change is in the "other
errors" that don't arise in normal use.