Eight points is HUGE. Most polls are around 2-3 percent margin of error. Their polls were showing triple that, and calling for the wrong party to win, in a not-terribly-close election (11 million to 9 million). That's far from "within the margin of error".
No, look, that 3% is in either direction for each number. (Actually very few polls are down to 2%, more likely up to 4-5%.)
So the Labour result was very likely within the MOE, but the Conservative result would depend on the size of the poll.
Look at this tracker of poll averages—they were really dead even at the end:
http://www.telegraph.co.uk/news/general-election-2015/11374181/latest-poll-tracker.html