Throw EngineClosedException if engine is closed

Currently we throw misleading exception in acquireSearcher
if we try to acquire while we are failing the engine. We should
throw an EngineClosedException instead.

Relates to #5633

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

@@ -663,6 +663,7 @@ public final Searcher acquireSearcher(String source) throws EngineException {
             return newSearcher(source, searcher, manager);
         } catch (Throwable ex) {
             logger.error("failed to acquire searcher, source {}", ex, source);
+            ensureOpen(); // throw EngineCloseException here if we are already closed
             throw new EngineException(shardId, ex.getMessage());
         }
     }