I'm a bit uncomfortable with the stark, binary sort of way of viewing success. I think it is more nuanced than that, as there are various factors in play.
Obviously we won the CL, so that is successful in anyone's estimation. But let's imagine we didn't overcome the deficit against Barcelona and we got knocked out in the semi final. And let's imagine the Prem went just as it did, and we got a record points haul of 97 points, an incredible tally, only to be edged out by the oil cheats at Man City, who got 98 points.
I know it's hypothetical, but if both of those scenarios happened, I would be loathe to call it an unsuccessful season.
In a binary sort of approach, win/lose, then yes, definitely unsuccessful. But I think you have to look at all the factors around it.