Depends on the situation really. Failure by itself on a check shouldn't necessarily incur death, but a failure and Despair? Most probably. That is the reason those red dice exist, to more consistently challenge the players while providing consequence. Those consequence can occasionally kill a player if the situation is right and more often then not; they won't be able to get out of that situation by themselves.
The meg-train for example, object retrieved, two rounds before you reach the imperial base. A person attempts a jump and fails with despair (environmental factors, and the GM spent a pip, just to really spice things up.). This particular character wasn't carrying the artefact, however just dropping his blaster doesn't really have any impact so the GM decides to do something cool.
"The train shudders as the player is about to jump and the character, losing his footing begins to topple over the edge; all he can see is the racing rocks below and the sleek smooth surface of the train as he begins to fall towards certain death..."
Boom the consequence is established, they are in a situation that will absolutely kill them if they aren't saved at this moment. At this point they cannot save themselves.
"Crap! My marauder does a siding dive and grabs out to catch him! What wo"
"Interesting, roll an athletics check, I probably would say that it's a 2 purple check, but the fact you didn't expect it coming would add 2 setback dice to that pool. Just to keep this interesting..." *spends a destiny point to increase the roll once the player has committed to rolling.
Of course, I would only allow one player to do this; with one player to "aid" the check. I am very keen on enforcing one check, one character rolls for it. At this point even if I rolled a second despair I wouldn't then cause the second character to fall off; but rather lose something else. in the process of doing so; his blaster rifle, a data card, a lightsaber and so fourth.
"You succeed! You just manage to grab his arm firmly in your grasp, you see your blaster rifle tumble past you into the void but you manage to grab and haul that character up. With the advantages however, the pilot seeing this jockeys up closer to the train then he initially dared; making subsequent checks to get aboard a little easier..."
or
"You are just moments too late as you grasp thin air, you only get a moment to see the smuggler get lost in the rush of rocks below. You don't have time to mourn your loss however as the base begins to loom in the distance; you have to get off this train!"
What I did there was put a character into a situation that they alone couldn't hope to escape from and by allowing the players to save one another greatly raises the emotional stakes and the potential for good story telling, even if failed? it would have generated awesome stories that would last a lifetime and hopefully allow a narrative to be built from character cooperation. Of course it depends on the millage of your game; I personally get fairly bored if I don't get to gamble. I am very much a story driven player; but sometimes there has to be consquences that make sense within the narrative of the tale. Sometimes, passing the torch is how the story lives on.
But what if they don't help?
"The character topples overboard as you stand motionless. You all know within that moment if any of you had done anything, you might've had a chance of saving him. Any force sensitives in this party will take 10 conflict for knowing inaction. After all, he was your friend, wasn't he?"
Sure, I imagine many of you would've given the unlucky one chance to save himself, but I don't believe in that. I believe character's are only capable of so much individually. That and it really creates dramatic tension between players; is your the kind of person who would attempt to rescue a character whom was verbally hostile to you earlier? Or is this your chance to betray the captain of your team for a promotion? Of course I would recommend talking about the tone of your game first, that's just how I would envision such a situation playing out.
Also, there are those "stupid" checks that are plainly impossible, jumping a 50 foot gap that's narratively too big? "That's a impossible check. But if you spend that destiny point to even attempt it? Sure. 5 difficulty dice, Given the high winds, I would add 3 setback dice. And you know what? I'll even spend the destiny point on this one. Just be certain that if you attempt this check and fail, you will die. Despairs here would just kill you whether you succeed or fail." be blunt, be honest and let them trust in you that you do have a narrative that will get them where they want/need to be; just sometimes it takes a bit more patience.