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>
a67e326cb9