[FEATURE] Unclear PPL error message: Empty mapping on wildcard indices

Is your feature request related to a problem?
Suppose a cluster has multiple indices with incompatible mappings, and both are matched by the same wildcard:

• idx-a -> Mapping A
• idx-b -> Mapping B

source = idx-* then returns the error: Failed to read mapping for index pattern [[Ljava.lang.String;@9acdf8].

This error is because no mapping is returned by the mapping endpoint.

Here's a real-world example, checking a specific OCSF index in a cluster with many such indices:

% curl 'ocsf-1.1.0-2004-detection_finding-2025.03.20-000034/_mapping' | jq .
{
  "ocsf-1.1.0-2004-detection_finding-2025.03.20-000034": {
    "mappings": {
      // Redacted
    }
}

But trying to get all OCSF indices returns an empty mapping:

% curl 'oscf-1.1.0*/_mapping'
{}

What solution would you like?
Clarify the error message. Something like Found an invalid index mapping for 'ocsf-1.1.0*'. Check that all indices matching the pattern share a valid mapping.

What alternatives have you considered?
N/A

Do you have any additional context?
Internal tracking (for AWS employees): https://t.corp.amazon.com/P343512413