dex: burn rounding error rather than halting the chain

[hdevalence]

After running these commands, similar to our test code:

cargo run --release --bin pcli -- -n http://localhost:8080 tx position order sell 25gn@1gm
cargo run --release --bin pcli -- -n http://localhost:8080 tx position order buy 1pusd@20gm
cargo run --release --bin pcli -- -n http://localhost:8080 tx position order buy 1pusd@20gm
cargo run --release --bin pcli -- -n http://localhost:8080 tx position order buy 5penumbra@1gm
cargo run --release --bin pcli -- -n http://localhost:8080 tx position order sell 1pusd@5penumbra
cargo run --release --bin pcli -- -n http://localhost:8080 tx swap 10penumbra --into gn

I saw this chain halt: https://gist.github.com/hdevalence/2ee2c17d6092dca701b45821b2cc9b0f

This commit changes the code to instead burn any rounding error and warn loudly about it. (We should also figure out what the cause is and fix that).