HDFS-16298. Improve error msg for BlockMissingException (#3615)

Reviewed-by: Hui Fei <ferhui@apache.org>

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java

@@ -1003,7 +1003,7 @@ public class DFSInputStream extends FSInputStream
       String description = "Could not obtain block: " + blockInfo;
       DFSClient.LOG.warn(description + errMsg
           + ". Throwing a BlockMissingException");
-      throw new BlockMissingException(src, description,
+      throw new BlockMissingException(src, description + errMsg,
           block.getStartOffset());
     }