HDFS-16515. Improve ec exception message

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSDirErasureCodingOp.java

@@ -168,8 +168,8 @@ private static List<XAttr> setErasureCodingPolicyXAttr(final FSNamesystem fsn,
       throw new FileNotFoundException("Path not found: " + srcIIP.getPath());
     }
     if (!inode.isDirectory()) {
-      throw new IOException("Attempt to set an erasure coding policy " +
-          "for a file " + src);
+      throw new IOException("Cannot set an erasure coding policy " +
+          "for a file " + src + ". Expected to be a directory");
     }
 
     final XAttr ecXAttr;

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestUnsetAndChangeDirectoryEcPolicy.java

@@ -362,8 +362,8 @@ public void testEcPolicyOnFile() throws Exception {
       fs.setErasureCodingPolicy(ecFilePath, ecPolicy.getName());
       fail("IOException should be thrown for setting EC policy on file");
     } catch (IOException e) {
-      assertExceptionContains("Attempt to set an erasure coding policy " +
-          "for a file " + ecFilePath, e);
+      assertExceptionContains("Cannot set an erasure coding policy " +
+          "for a file " + ecFilePath + ". Expected to be a directory", e);
     }
 
     // Unset EC policy on file