Add basic CFError Display and Error impls

Part of #692.

framework-crates/objc2-core-foundation/src/error.rs

@@ -0,0 +1,14 @@
+#![cfg(all(feature = "CFBase", feature = "CFString"))]
+use core::fmt;
+
+use crate::{CFError, CFErrorCopyDescription};
+
+impl fmt::Display for CFError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let desc = unsafe { CFErrorCopyDescription(self) }.unwrap();
+        write!(f, "{desc}")
+    }
+}
+
+#[cfg(feature = "std")] // use core::error::Error from Rust 1.81 once in MSRV.
+impl std::error::Error for CFError {}

framework-crates/objc2-core-foundation/src/lib.rs

@@ -29,6 +29,8 @@ mod cf_type;
 mod data;
 #[cfg(feature = "CFDate")]
 mod date;
+#[cfg(feature = "CFError")]
+mod error;
 mod generated;
 #[cfg(feature = "CFCGTypes")]
 mod geometry;