webassembly/proxy_c: Ensure objs thrown into generators are exceptions.

This commit defines a new `JsException` exception type which is used on the
Python side to wrap JavaScript errors.  That's then used when a JavaScript
Promise is rejected, and the reason is then converted to a `JsException`
for the Python side to handle.

This new exception is exposed as `jsffi.JsException`.

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

tests/ports/webassembly/await_error_handling.mjs

@@ -0,0 +1,22 @@
+// Test await'ing on a JavaScript async function that throws a JavaScript Error.
+
+const mp = await (await import(process.argv[2])).loadMicroPython();
+
+globalThis.foo = async () => {
+    console.log(2);
+    throw Error("test");
+};
+
+await mp.runPythonAsync(`
+import js, jsffi
+print(1)
+try:
+    await js.foo()
+except jsffi.JsException as er:
+    error = er
+print(error)
+print(3)
+`);
+
+const error = mp.globals.get("error");
+console.log(error instanceof Error, error.name, error.message);

tests/ports/webassembly/await_error_handling.mjs.exp

@@ -0,0 +1,5 @@
+1
+2
+(<JsProxy 6>, 'Error', 'test')
+3
+true Error test