Improve error message when calling json on void function

[agostbiro]

When calling json() on the execution result of function that doesn't return anything, the following error is generated:

Error: DataConversion

Caused by:
    EOF while parsing a value at line 1 column 0

We should catch this and create a better error message for the user that explains the problem.

Minimal code to reproduce assuming a contract with a new method that doesn't return anything:

let worker = workspaces::sandbox().await?;
let wasm = fs::read(/*WASM FILEPATH*/)?;
let contract = worker.dev_deploy(&wasm).await?;
contract.call("new").transact().await?.json()?;