fix: add error details for INTERNAL error (#2413)

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Fixes #<issue_number_goes_here> ☕️

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https://togithub.com/GoogleCloudPlatform/java-docs-samples/blob/main/SAMPLE_FORMAT.md).

google-cloud-spanner-executor/src/main/java/com/google/cloud/executor/spanner/CloudExecutor.java

@@ -428,7 +428,8 @@ protected Status toStatus(SpannerException e) {
       case CANCELLED:
         return Status.fromCode(Status.CANCELLED.getCode()).withDescription(e.getMessage());
       case INTERNAL:
-        return Status.fromCode(Status.INTERNAL.getCode()).withDescription(e.getMessage());
+        return Status.fromCode(Status.INTERNAL.getCode())
+            .withDescription(e.getMessage() + e.getReason() == null ? "" : ": " + e.getReason());
       case FAILED_PRECONDITION:
         return Status.fromCode(Status.FAILED_PRECONDITION.getCode())
             .withDescription(e.getMessage());