Improve diagnostics with missing trait implementations

Rejigger a few spans, work around an odd rustc issue, and hopefully
produce higher quality error messages!

Closes #1528

crates/macro/ui-tests/missing-catch.stderr

@@ -1,7 +1,7 @@
 error[E0277]: the trait bound `std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>: wasm_bindgen::convert::traits::FromWasmAbi` is not satisfied
- --> $DIR/missing-catch.rs:3:1
+ --> $DIR/missing-catch.rs:6:9
   |
-3 | #[wasm_bindgen]
-  | ^^^^^^^^^^^^^^^ the trait `wasm_bindgen::convert::traits::FromWasmAbi` is not implemented for `std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>`
+6 |     pub fn foo() -> Result<JsValue, JsValue>;
+  |            ^^^ the trait `wasm_bindgen::convert::traits::FromWasmAbi` is not implemented for `std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>`
 
 For more information about this error, try `rustc --explain E0277`.

crates/macro/ui-tests/traits-not-implemented.rs

@@ -0,0 +1,11 @@
+use wasm_bindgen::prelude::*;
+
+struct A;
+
+#[wasm_bindgen]
+extern "C" {
+    #[wasm_bindgen]
+    pub fn foo(a: A);
+}
+
+fn main() {}

crates/macro/ui-tests/traits-not-implemented.stderr

@@ -0,0 +1,7 @@
+error[E0277]: the trait bound `A: wasm_bindgen::convert::traits::IntoWasmAbi` is not satisfied
+ --> $DIR/traits-not-implemented.rs:8:12
+  |
+8 |     pub fn foo(a: A);
+  |            ^^^ the trait `wasm_bindgen::convert::traits::IntoWasmAbi` is not implemented for `A`
+
+For more information about this error, try `rustc --explain E0277`.

crates/macro/ui-tests/unknown-type-in-import.rs

@@ -0,0 +1,10 @@
+use wasm_bindgen::prelude::*;
+
+#[wasm_bindgen]
+extern "C" {
+    #[wasm_bindgen]
+    pub fn foo(a: A);
+}
+
+fn main() {}
+

crates/macro/ui-tests/unknown-type-in-import.stderr

@@ -0,0 +1,7 @@
+error[E0412]: cannot find type `A` in this scope
+ --> $DIR/unknown-type-in-import.rs:6:19
+  |
+6 |     pub fn foo(a: A);
+  |                   ^ not found in this scope
+
+For more information about this error, try `rustc --explain E0412`.