New extender for error handling

[franzliedke]

Fixes #1781

This extender implements several methods for extending the new error
handling stack implemented in #1843.

Most use-cases should be covered, but I expect some challenges for more
complex setups. We can tackle those once they come up, though. Basic
use-cases should be covered.

Confirmed

• Tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).

[franzliedke]

Tagging some people:

• @matteocontrini because you reported the original issue
• @datitisev because you maintain fof/sentry if I remember correctly
• @clarkwinkelmann because you've created a bunch of extenders already

[matteocontrini]

So taking in consideration the use case mentioned in the issue the usage of this extender would be something like:

return [
    (new Extend\ErrorHandling)
        ->handler(MyException::class, MyExceptionHandler::class)
];

Where MyExceptionHandler exposes an handle method. Is that correct?

Shouldn't there be an interface for exception handlers, perhaps?

[franzliedke]

@matteocontrini That depends on the exact use-case. If all you're doing is configuring the status code for an exception, you should use the other methods.

Handlers are really only necessary for adding more details to error output (in the JSON-API layer). #1843 removed most of our layers, and only two remain - those embed validation errors into the JSON-API response. So, assuming you're doing something simpler than that, please try out the other methods.

Shouldn't there be an interface for exception handlers, perhaps?

I thought about it and it does not help much, as that would mean you cannot type hint the argument to the handle() class.

[dsevillamartin]

PHPDoc needed for handler method - probably including warning that one of the other methods should work instead or something. And perhaps specify what instances it would be used and why the others don't help in those... if that makes sense.

[franzliedke]

You're right, thanks! Added another docblock.

[dsevillamartin]

LGTM!