message: Add note why panic is safe
diff: @@ -307,7 +307,12 @@ impl<A: BackendApi, S: Storage, Q: Querier> Environment<A, S, Q> {
  .memories()
  .next()
  .map(|pair| pair.1.clone());
  - let memory = first.unwrap();
  + // Every contract in CosmWasm must have exactly one exported memory.
  + // This is ensured by `check_wasm`/`check_wasm_memories`, which is called for every
  + // contract added to the Cache as well as in integration tests.
  + // It is possible to bypass this check when using `Instance::from_code` but then you
  + // learn the hard way when this panic or when trying to upload the contract to chain.
  + let memory = first.expect("A contract must have exactly one exported memory.");
  Ok(memory)
  })
  .expect("Wasmer instance is not set. This is a bug in the lifecycle.")
  