Guess root cause support unwrap

ElasticsearchException.guessRootCauses would return wrapper exception if
inner exception was not an ElasticsearchException. Fixed to never return
wrapper exceptions.

Relates elastic#50417

server/src/main/java/org/elasticsearch/ElasticsearchException.java

@@ -641,7 +641,7 @@ public static ElasticsearchException[] guessRootCauses(Throwable t) {
                 }
             }
         }
-        return new ElasticsearchException[]{new ElasticsearchException(t.getMessage(), t) {
+        return new ElasticsearchException[]{new ElasticsearchException(ex.getMessage(), ex) {
             @Override
             protected String getExceptionName() {
                 return getExceptionName(getCause());

server/src/test/java/org/elasticsearch/ElasticsearchExceptionTests.java

@@ -163,6 +163,16 @@ public void testGuessRootCause() {
             assertEquals("illegal_argument_exception", foobars[0].getExceptionName());
         }
 
+        {
+            final ElasticsearchException[] foobars = ElasticsearchException.guessRootCauses(
+                new RemoteTransportException("abc", new IllegalArgumentException("foobar")));
+            assertEquals(foobars.length, 1);
+            assertThat(foobars[0], instanceOf(ElasticsearchException.class));
+            assertEquals("foobar", foobars[0].getMessage());
+            assertEquals(IllegalArgumentException.class, foobars[0].getCause().getClass());
+            assertEquals("illegal_argument_exception", foobars[0].getExceptionName());
+        }
+
         {
             XContentParseException inner = new XContentParseException(null, "inner");
             XContentParseException outer = new XContentParseException(null, "outer", inner);