Index Management: Do not fail on missing "@custom" ingest pipeline

[flash1293]

Similar to #183987

Integrations reference a @custom pipeline for every datastream which allows the user to make adjustments to the data collection without interfering with the package-provided pipelines.

However, by default these pipelines are not created - so navigating there will error out:

As it is expected these @Custom pipelines don't exist, the user should be sent to an empty form with the name of the pipeline prefilled instead of an error message, so they can create the pipeline.

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)

[alisonelizabeth]

As it is expected these @Custom pipelines don't exist, the user should be sent to an empty form with the name of the pipeline prefilled instead of an error message, so they can create the pipeline.

Can we rely on this? ie if a @custom- pipeline doesn't exist, always fall back to the create form?

[flash1293]

IMHO it could even be a general thing - maybe it could show a warning "pipeline not found, clear here to create"?

The thing that's important to me is that the user always has a way forward.