MINOR: improve logging for Kafka Streams stateful task management (apache#20942)

Changing log level from ERROR to WARN, as it's expected that this can
happen, and we should not incorrectly make users worry about it.

Reviewers: PoAn Yang <[email protected]>, Colt McNealy
 <[email protected]>, Lucas Brutschy <[email protected]>,
 Chia-Ping Tsai <[email protected]>

Author: mjsax

streams/src/main/java/org/apache/kafka/streams/processor/internals/StateManagerUtil.java

@@ -147,7 +147,7 @@ static void closeStateManager(final Logger log,
                     }
                 }
             } else {
-                log.error("Failed to acquire lock while closing the state store for {} task {}", taskType, id);
+                log.warn("Unable to acquire lock while closing the state store for {} task {}", taskType, id);
             }
         } catch (final IOException e) {
             final ProcessorStateException exception = new ProcessorStateException(