Fix PHPdoc for error handlers in ErrorMiddleware

[TiMESPLiNTER]

This PR adds type hints for string beside callable and ErrorHandlerInterface as the $this->callableResolver->resolve can resolve strings if they are a registered service id in the di container.

[coveralls]

Coverage remained the same at 100.0% when pulling 220a812 on TiMESPLiNTER:fix/errorhandler-middleware-phpdoc into 5613cbb on slimphp:4.x.

[l0gicgate]

I'm curious, is this necessary since a callable can in fact be a string?

[TiMESPLiNTER]

I'm curious, is this necessary since a callable can in fact be a string?

If the string is a callable function, you're right yes. But a container id is not a callable but just a string which later on gets resolved to a callable.

Valid callable

function myHandler() { }
$errorMiddleware->setErrorHandler(HttpException::class, 'myHandler', true);

Invalid callable

$errorMiddleware->setErrorHandler(HttpException::class, 'my_handlers_container_id', true);

[l0gicgate]

Fair enough. Merging.