webassembly/proxy_c: Reject promises with a PythonError instance.

The `reason` in a rejected promise should be an instance of `Error`.  That
leads to better error messages on the JavaScript side.

Signed-off-by: Damien George <damien@micropython.org>

ports/webassembly/proxy_c.c

@@ -297,7 +297,14 @@ EM_JS(void, js_then_resolve, (uint32_t * ret_value, uint32_t * resolve, uint32_t
 });
 
 EM_JS(void, js_then_reject, (uint32_t * ret_value, uint32_t * resolve, uint32_t * reject), {
-    const ret_value_js = proxy_convert_mp_to_js_obj_jsside(ret_value);
+    // The ret_value object should be a Python exception.  Convert it to a
+    // JavaScript PythonError and pass it as the reason to reject the promise.
+    let ret_value_js;
+    try {
+        ret_value_js = proxy_convert_mp_to_js_obj_jsside(ret_value);
+    } catch(error) {
+        ret_value_js = error;
+    }
     const resolve_js = proxy_convert_mp_to_js_obj_jsside(resolve);
     const reject_js = proxy_convert_mp_to_js_obj_jsside(reject);
     reject_js(ret_value_js);
@@ -359,7 +366,7 @@ static mp_obj_t proxy_resume_execute(mp_obj_t self_in, mp_obj_t send_value, mp_o
     } else { // ret_kind == MP_VM_RETURN_EXCEPTION;
         // Pass the exception through as an object to reject the promise (don't raise/throw it).
         uint32_t out_ret_value[PVN];
-        proxy_convert_mp_to_js_obj_cside(ret_value, out_ret_value);
+        proxy_convert_mp_to_js_exc_cside(ret_value, out_ret_value);
         js_then_reject(out_ret_value, out_resolve, out_reject);
         return mp_const_none;
     }

tests/ports/webassembly/run_python_async_error.mjs

@@ -0,0 +1,11 @@
+// Test raising an exception in async Python code running in runPythonAsync,
+// that the JavaScript-level promise is rejected with a PythonError.
+
+const mp = await (await import(process.argv[2])).loadMicroPython();
+
+try {
+    await mp.runPythonAsync("await fail");
+} catch (error) {
+    console.log(error.name, error.type);
+    console.log(error.message);
+}

tests/ports/webassembly/run_python_async_error.mjs.exp

@@ -0,0 +1,5 @@
+PythonError NameError
+Traceback (most recent call last):
+  File "<stdin>", line 1, in <module>
+NameError: name 'fail' isn't defined
+