[Transform] Log failure to get config as warning if not permanent

When a transform that's just starting fails to get its config
this is always logged as an error, but only causes the transform
to fail if the reason for it is that something doesn't exist.
If the exception was just a transient search failure then the
problem is considered non-fatal, and can be logged as a warning
rather than an error.

x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/transforms/TransformIndexer.java

@@ -343,12 +343,13 @@ protected void onStart(long now, ActionListener<Boolean> listener) {
                     }
                 }, failure -> {
                     String msg = TransformMessages.getMessage(TransformMessages.FAILED_TO_RELOAD_TRANSFORM_CONFIGURATION, getJobId());
-                    logger.error(msg, failure);
                     // If the transform config index or the transform config is gone, something serious occurred
                     // We are in an unknown state and should fail out
                     if (failure instanceof ResourceNotFoundException) {
+                        logger.error(msg, failure);
                         reLoadFieldMappingsListener.onFailure(new TransformConfigLostOnReloadException(msg, failure));
                     } else {
+                        logger.warn(msg, failure);
                         auditor.warning(getJobId(), msg);
                         reLoadFieldMappingsListener.onResponse(null);
                     }