Avoid class cast exception from index writer (#28989)

When an index writer encounters a tragic exception, it could be a
Throwable and not an Exception. Yet we blindly cast the tragic exception
to an Exception which can encounter a ClassCastException. This commit
addresses this by checking if the tragic exception is an Exception and
otherwise wrapping the Throwable in a RuntimeException if it is not. We
choose to wrap the Throwable instead of passing it around because
passing it around leads to changing a lot of places where we handle
Exception to handle Throwable instead. In general, we have tried to
avoid handling Throwable and instead let those bubble up to the uncaught
exception handler.

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -1730,7 +1730,13 @@ public class InternalEngine extends Engine {
         // we need to fail the engine. it might have already been failed before
         // but we are double-checking it's failed and closed
         if (indexWriter.isOpen() == false && indexWriter.getTragicException() != null) {
-            failEngine("already closed by tragic event on the index writer", (Exception) indexWriter.getTragicException());
+            final Exception tragicException;
+            if (indexWriter.getTragicException() instanceof Exception) {
+                tragicException = (Exception) indexWriter.getTragicException();
+            } else {
+                tragicException = new RuntimeException(indexWriter.getTragicException());
+            }
+            failEngine("already closed by tragic event on the index writer", tragicException);
             engineFailed = true;
         } else if (translog.isOpen() == false && translog.getTragicException() != null) {
             failEngine("already closed by tragic event on the translog", translog.getTragicException());