Enhancement: Get User on non existant userId

[evinkuraga]

If the Chatkit::users()->show($userId) method is attempted on a user that either no longer exists or wasn't created, it crashes the app with an error instead of returning an empty object or an error code. Perhaps this was intentional, but if it was an oversight, I think it would be better to just return an object with the error code or error message.

[pascalboucher]

@evinkuraga hmm right, my approach there was to let chatkit handle the validations and throw it's exceptions when needed, but you are right, I think it would be more appropriate on certain occasion to return false for example.

I'll look into it next week.

Or feel free to add a PR if you have time.