Improve error message for non-copy struct fields

Make sure the error message points to the type in question instead of to
the `#[wasm_bindgen]` macro which can be overly confusing!

crates/macro/ui-tests/pub-not-copy.rs

@@ -0,0 +1,10 @@
+#![crate_type = "rlib"]
+
+extern crate wasm_bindgen;
+
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+pub struct A {
+    pub field: String,
+}