Factor attr parsing into serde_item crate

serde_item/src/error.rs

@@ -0,0 +1,19 @@
+use std::error;
+use std::fmt;
+
+#[derive(Debug)]
+pub enum Error {
+    UnexpectedItemKind,
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "expected a struct or enum")
+    }
+}
+
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        "expected a struct or enum"
+    }
+}