[wasmtime-api] Record original Trap from API callback. (#657)

* Record original Trap from API callback.

Fixes #645

* use TrapRegistry

* comment about magic number

crates/api/tests/traps.rs

@@ -0,0 +1,49 @@
+use std::rc::Rc;
+use wasmtime::*;
+use wat::parse_str;
+
+#[test]
+fn test_trap_return() -> Result<(), String> {
+    struct HelloCallback;
+
+    impl Callable for HelloCallback {
+        fn call(&self, _params: &[Val], _results: &mut [Val]) -> Result<(), HostRef<Trap>> {
+            Err(HostRef::new(Trap::new("test 123".into())))
+        }
+    }
+
+    let engine = HostRef::new(Engine::default());
+    let store = HostRef::new(Store::new(&engine));
+    let binary = parse_str(
+        r#"
+                (module
+                (func $hello (import "" "hello"))
+                (func (export "run") (call $hello))
+                )
+            "#,
+    )
+    .map_err(|e| format!("failed to parse WebAssembly text source: {}", e))?;
+
+    let module = HostRef::new(
+        Module::new(&store, &binary).map_err(|e| format!("failed to compile module: {}", e))?,
+    );
+    let hello_type = FuncType::new(Box::new([]), Box::new([]));
+    let hello_func = HostRef::new(Func::new(&store, hello_type, Rc::new(HelloCallback)));
+
+    let imports = vec![hello_func.into()];
+    let instance = Instance::new(&store, &module, imports.as_slice())
+        .map_err(|e| format!("failed to instantiate module: {}", e))?;
+    let run_func = instance.exports()[0]
+        .func()
+        .expect("expected function export");
+
+    let e = run_func
+        .borrow()
+        .call(&[])
+        .err()
+        .expect("error calling function");
+
+    assert_eq!(e.borrow().message(), "test 123");
+
+    Ok(())
+}