[ML] Add explanation so far to file structure finder exceptions (#38337)

The explanation so far can be invaluable for troubleshooting
as incorrect decisions made early on in the structure analysis
can result in seemingly crazy decisions or timeouts later on.

Relates elastic/kibana#29821

x-pack/plugin/ml/src/main/java/org/elasticsearch/xpack/ml/filestructurefinder/FileStructureFinderManager.java

@@ -7,6 +7,7 @@
 
 import com.ibm.icu.text.CharsetDetector;
 import com.ibm.icu.text.CharsetMatch;
+import org.elasticsearch.ElasticsearchException;
 import org.elasticsearch.ElasticsearchTimeoutException;
 import org.elasticsearch.common.collect.Tuple;
 import org.elasticsearch.common.unit.TimeValue;
@@ -148,6 +149,14 @@ public FileStructureFinder findFileStructure(List<String> explanation, int ideal
                 Math.max(MIN_SAMPLE_LINE_COUNT, idealSampleLineCount), timeoutChecker);
 
             return makeBestStructureFinder(explanation, sampleInfo.v1(), charsetName, sampleInfo.v2(), overrides, timeoutChecker);
+        } catch (Exception e) {
+            // Add a dummy exception containing the explanation so far - this can be invaluable for troubleshooting as incorrect
+            // decisions made early on in the structure analysis can result in seemingly crazy decisions or timeouts later on
+            if (explanation.isEmpty() == false) {
+                e.addSuppressed(
+                    new ElasticsearchException(explanation.stream().collect(Collectors.joining("]\n[", "Explanation so far:\n[", "]\n"))));
+            }
+            throw e;
         }
     }