Better declarative config error messages

When an error occurs in declarative config, the error message should tell you the location in the document where the error occurred.

Currently, we only give generic errors of the form "batch span processor exporter is required but is null". These messages can be ambiguous since its possible to have multiple processors. Even when unambiguous, they're more difficult to parse and fix than something that includes document context like: .tracer_provider.processors[0].batch exporter is required but is null.

Locate the declarative config validation path that produces the generic error about a null batch span processor exporter, then inspect how document locations are represented during validation. Done means errors include the relevant document path, such as .tracer_provider.processors[0].batch, while retaining the existing validation context.