allow from variants to skip the error attr's message

[onkoe]

in other words, make this valid:

use pisserror::Error;
use std::error::Error;

#[derive(Debug, Error)]
enum MyError {
    #[error]
    VariantWithFromField(#[from] std::io::Error),
    #[error("message has to be here: {_0}")]
    NormalVariant(i32),
}

[onkoe]

nope. let's do #17