No matter how bad a player does, I've never found it productive to go berserk over them. Most of the time (90%) they just need a bit of instruction, and 5% of the time they're either having a bad day or just messed up. It's only when a player purposely AFKs or whines on and on about nothing that I get very frustrated.

I've had great luck on DF, but that's usually because I'm lead tank and just say "no worries" or "no sweat, tho maybe next time you should" when something small goes wrong. Generally if one person speaks softly first, anyone wanting to freak out stays their words. At least in my experience