fix: Async logging should not rethrow error in onFailure callback (#923)

Author: losalex

google-cloud-logging/src/main/java/com/google/cloud/logging/LoggingImpl.java

@@ -882,7 +882,7 @@ public void flush() {
     try {
       ApiFutures.allAsList(writesToFlush).get(FLUSH_WAIT_TIMEOUT_SECONDS, TimeUnit.SECONDS);
     } catch (InterruptedException | ExecutionException | TimeoutException e) {
-      throw new RuntimeException(e);
+      System.err.println("ERROR: flush failure: " + e);
     }
   }