Add flag for errors backtraces

[ethanfrey]

The contracts in cosmwasm repo have:

[features]
backtraces = ["cosmwasm-std/backtraces", "cosmwasm-vm/backtraces"]

With cargo +nightly test --features backtraces we can get full backtraces on any any unexpected error that shows up in our unit tests. Let's expose this in these contracts too.

[ethanfrey]

I checked the contracts, they do actually have this already.