add is_not_found functor to error; tweak status code matcher

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/result/not_found.rs

@@ -0,0 +1,12 @@
+use super::Result;
+use crate::{error, Error};
+
+pub trait NotFound<T> {
+	#[must_use]
+	fn is_not_found(&self) -> bool;
+}
+
+impl<T> NotFound<T> for Result<T, Error> {
+	#[inline]
+	fn is_not_found(&self) -> bool { self.as_ref().is_err_and(error::is_not_found) }
+}