Here's my suggestion. It worked a treat with my children and I've demonstrated it to others with similar success ...
Create a diversion by pulling a surprised face and saying "Show me your leg. Did a dog bite you? No, a dog didn't bite you. Show me your hand. Do you have yucky blood on your finger? No, you don't have a sore finger."
You can carry this on with different parts of their body for as long as it takes to get their attention, adding a few tickles along the way while looking for the injury that must be making them cry. As long as you're in private you could add "Have you got ants in your pants that are biting your bottom?" Encourage them to help you look at the part of their body you are mentioning.
Once they've calmed down, say "You can cry if a dog bites you. And you can cry if you have ants in your pants that are biting your bottom ... but you don't cry now. Crying now is silly."
If they start screaming again it is time to say "No ... crying now is silly. We don't want to listen to silly crying," and leave them without an audience.
If it happens again, repeat the process. You'll be surprised how quickly even a young child gets the message. It didn't take long before I could pull a surprised face and ask "Did a dog bite you?" or "Is this silly crying?" and my child would stop immediately. I'd reward them with "We don't want silly crying, do we? Come and give me a cuddle."
I think many people underestimate the intelligence of two-year-olds and their ability to learn. I also suspect many parents look at their screaming four-year-olds and regret not addressing the issue effectively when they were two. I believe it is best to wait until you get positive behaviour before you offer any kind of reward. The trick is to make them laugh along the way and reinforce what you consider to be appropriate and inappropriate behaviour so they learn what you expect from them.
Good luck. :)