Allow errorHandlerSupplier to re-init

[solidline]

We noticed @ExceptionHandler annotated methods are not getting picked up in our application. After debugging this is because GraphQLErrorStartupListener only initializes the errorHandlerSupplier on the first ContextRefreshedEvent event. This is an issue because the application has not registered all beans. By removing the if statement we now see all factories inside GraphQLErrorFromExceptionHandler

[Sam-Kruglov]

I just checked myself and this is called about 10+ times during startup.
As I wrote in #309, this is basically going to call ReflectiveMethodValidator#isGraphQLExceptionHandler for each event.getApplicationContext().getBeanDefinitionNames()

The first couple of times it has around 20 beans in my case and the last call has over 1k beans returned. Each this bean is traversed using reflection, so I think that's not the best approach here.

I tried debugging for ApplicationReadyEvent and that was only called once with all the 1k beans. So, perhaps, changing the event to that one would do the trick?

[solidline]

looks to work as expected with the ApplicationReadyEvent i've updated the PR to reflect the change.

[oliemansm]

Thanks!