warn when default provider overrides custom provider.

### Which @angular/* package(s) are relevant/related to the feature request?

_No response_

### Description

I spent some time debugging why my custom ErrorHandler wasn't being provided, it turns out it was being overridden by a default provider further down in the provider list. It would be nice to have a warning in this situation.

### Proposed solution

n/a

### Alternatives considered

n/a

The issue does not name a package, file, test, or entry point. Start by tracing Angular's provider-resolution path for a custom ErrorHandler and the default provider, then review the comment thread for the intended warning behavior. Done means an agreed warning is implemented and covered for the reported override scenario.

Written by the indexing model from the issue text.