Skip stacktrace in info log for s3 region loading issue

The stacktrace is rather excessive given the level of the log message.
The information provided in the exception message should be sufficient
for taking actions. Also adjust the wording to better match the
contribuation guide for logging messages.

Author: ywangd

modules/repository-s3/src/main/java/org/elasticsearch/repositories/s3/S3RepositoryPlugin.java

@@ -101,7 +101,7 @@ private static Region getDefaultRegion() {
         try {
             return DefaultAwsRegionProviderChain.builder().build().getRegion();
         } catch (Exception e) {
-            logger.info("failed to obtain region from default provider chain", e);
+            logger.info("unable to obtain region from default provider chain: {}", e.getMessage());
             return null;
         }
     }