rust: pin-init: add `#[default_error(<type>)]` attribute to initializer macros

rust: pin-init: add `#[default_error(<type>)]` attribute to initializer macros

The `#[default_error(<type>)]` attribute can be used to supply a default
type as the error used for the `[pin_]init!` macros. This way one can
easily define custom `try_[pin_]init!` variants that default to your
project specific error type. Just write the following declarative macro:

    macro_rules! try_init {
        ($($args:tt)*) => {
            ::pin_init::init!(
                #[default_error(YourCustomErrorType)]
                $($args)*
            )
        }
    }

Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Benno Lossin <lossin@kernel.org>