[Bug]: Explorer, fail more gracefully when attempting to delegate to an indexer that did not migrate to SubgraphService



### Description

This is what we currently get which is not very telling of what is wrong:

<img width="469" height="692" alt="Image" src="https://github.com/user-attachments/assets/4de64c05-b993-4f38-a76f-cee93f249c08" />

Ideally we should face gracefully or show a warning explaining this will fail.
