Fixed: Allow Listening to Subscription Errors

[7rakir]

Context
There are two overloaded methods for ExecutionDiagnosticEventListener.SubscriptionEventError.
One called with payload in Subscription.OnEvent; the other when enumerating subscriptions in SubscriptionEnumerable and SubscriptionEnumerator.

Users can override the former when deriving from ExecutionDiagnosticEventListener, but cannot override the latter.
If an exception happens during subscribing, AggregateExecutionDiagnosticEvents calls SubscriptionEventError(ISubscription, Exception). No listener deriving from ExecutionDiagnosticEventListener cannot override this and the call results in no-op.

Notes
I'm making only this small change for v15, because I understand ExecutionDiagnosticEventListener is being refactored for v16.

Tests
I haven't found any relevant tests for this particular functionality, but if they exist, please point them out to me.

[CLAassistant]

All committers have signed the CLA.

[michaelstaib]

@7rakir can you sign the CLA so we can merge your change?

[7rakir]

@7rakir can you sign the CLA so we can merge your change?

I might be doing something wrong, but:

• https://cla-assistant.io/ChilliCream/graphql-platform?pullRequest=8384 says "You have agreed to the CLA for ChilliCream/graphql-platform"
• https://cla-assistant.io/my-cla says "Everything OK!" next to "ChilliCream / hotchocolate"

I've clicked the "recheck" link in the CLAassistant comment.
I've revoked the CLA from the last year now and re-signed again, just to be sure. I guess it takes some time to propagate.

[michaelstaib]

@7rakir you are probably using different email addresses

[7rakir]

@7rakir you are probably using different email addresses

Correct as usual. Thank you. Should be OK now.