Death is just one consequence of failure. Having to rest early because of bad tactics is another type of failure.
If you use Death too often, it cheapens its meaning. Easy Deaths is what made spells like Raise Dead so important over the editions. It turned character death just in another type of "rest early". (Sometimes "rest early to prepare the Raise Dead spell", sometimes "let's get home so we find another trustworthy-enough looking fellow".
You might say "resting early" is not a real failure, because the consequences for the player are minimal. But so is the death of a character. You just wait for the raise dead or roll up a new character. You can still continue playing the game (and this might be one of the bigger differences to other games - losing doesn't mean you have to stop playing.), you're just using a changed or different game piece.
Bad decisions can lead to earlier rest, it can lead to another complication, it can lead to a failure in an character/adventure goal, or it can lead to death.
All these are valid failures. An EL = PL encounter in 3E or its 4E equivalent is the kind of encounter where bad decisions should lead to an earlier rest. It is a failure because the players notice that they screwed up and that they have to do better. Of course, if they even lose a PC, they will know that they screwed up even worse
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
, but that's not a requirement to make the player aware of failure or success.