[BUG] StreamOperatorService graph fails when StreamClasses are deleted (and not recreated)

[jrbentzon]

Description

When removing an existing StreamClass + CRD I've experienced the following error occur on the operator:

at Arcane.Operator.Services.Operators.StreamOperatorService.<>c__DisplayClass15_0.<Attach>b__0(Exception exception) in /app/Services/Operators/StreamOperatorService.cs:line 91
   at Akka.Streams.Implementation.Fusing.RecoverWith`2.Logic.OnFailure(Exception ex)
   at Akka.Streams.Implementation.Fusing.GraphInterpreter.Execute(Int32 eventLimit)
[14:54:04 ERR] Error in stage [Akka.Streams.Dsl.MergeHub`1+HubSink[Arcane.Operator.Models.Api.ResourceEvent`1[Arcane.Operator.Models.StreamDefinitions.Base.IStreamDefinition]]]: Upstream producer failed with exception, removing from MergeHub now
Akka.Streams.Dsl.MergeHub+ProducerFailed: Upstream producer failed with exception, removing from MergeHub now
 ---> k8s.Autorest.HttpOperationException: Operation returned an invalid status code 'NotFound', response body 404 page not found

Steps to reproduce the issue

1. Create a CRD + StreamClass Reference for MyStream
2. Deploy to cluster
3. Delete CRD + StreamClass

Describe the results you expected

Detects deletion of streamclass and feels fine about it not coming back...

System information

v0.0.7