Warn about double-inclusion of error message

I recently cooked up https://github.com/CraneStation/wasmtime/pull/532 which fixed a "double error message" getting printed for errors such as:

```rust
pub enum CompileError {
#[error("WebAssembly translation error: {0}")]
Wasm(#[from] WasmError),
}
```

It'd be great if this crate could somehow issue a diagnostic indicating that there's no need to include the `{0}` in the `#[error]` because it's already rendered in the final error via `#[from]`