rust: pin-init: examples: error: use `Error` in `fn main()`

When running this example with no cargo features enabled, the compiler
warns on 1.89:

error: struct `Error` is never constructed
--> examples/error.rs:11:12
|
11 | pub struct Error;
| ^^^^^
|
= note: `-D dead-code` implied by `-D warnings`
= help: to override `-D warnings` add `#[allow(dead_code)]`

Thus use the error in the main function to avoid this warning.

Signed-off-by: Benno Lossin <lossin@kernel.org>