Excess fields in Rust type causes hard panic

[jollygreenlaser]

• edgedb-tokio: 0.5
• edgedb-protocol: 0.6
• edgedb-derive: 0.5.1
• EdgeDB Version: EdgeDB Version: 4.4+596d0e4 on Ubuntu 22.04.3 LTS
• EdgeDB CLI Version: EdgeDB CLI 4.1.0+a8fe4d7

Steps to Reproduce:

struct Thing {
  a: String,
  b: String,
}

select {
  a := 'oops'
}

This will hard panic, completely killing the entire tokio/axum instance (not just the thread dealing with the request)

This should at worst give the same DescriptorMismatch as https://github.com/edgedb/edgedb-rust/issues/282 (and ideally should give something better, per that other issue)