[8.0] Rework HandlesOAuthErrors trait to middleware

[driesvints]

The benefit this gives is that it allows developers to overwrite the middleware and implement their own handling of errors if they want to. At the moment they were stuck with the implementation being a trait so the only way to overwrite the implementation was to rebind their own controllers.

With the new implementation you could bind a new middleware which extends the current one like so:

$this->app->bind(HandleOAuthErrors::class, function () {
    // return your own implementation...
});

At least it'll become way easier for developers to customize this than before. It also makes much more sense to handle the error handling in a middleware than through a trait. Given, the previous implementation was probably just an old one and just needed a refresh.

Added tests for the new middleware and removed the old one. Also removed the tests for the controllers (which aren't needed anymore now).

Sent this in to master because of the obvious breaking changes.

Fixes #289

[taylorotwell]

What will the upgrade path be for existing Passport users?

[TimWolla]

@driesvints Was this PR tested from within a Laravel app? We upgraded to dev-master passport within an Laravel 5.8 application to get PKCE support and noticed that OAuth errors result in a HTTP 500 now.

The issue being that Laravel itself catches Exceptions happening in the controller within the middleware pipeline (even in master of laravel/framework): https://github.com/laravel/framework/blob/9097786caae30e9ca243874e5c807609f1f2728e/src/Illuminate/Pipeline/Pipeline.php#L133, so they don't actually reach the HandleOAuthErrors middleware.

[driesvints]

@TimWolla hmm that's unfortunate. I don't have time myself to look at this atm so feel free to open up an issue for this.

[TimWolla]

@driesvints ack. I've filed #1062 to track this issue.