Defect Fixes and optimization for YANG translator.

Change-Id: I974a968f3c41e1abea9f2567aceb3d523645d0ae
diff --git a/src/main/java/org/onosproject/yangutils/utils/io/impl/YangFileScanner.java b/src/main/java/org/onosproject/yangutils/utils/io/impl/YangFileScanner.java
index 9de5358..bdf911a 100644
--- a/src/main/java/org/onosproject/yangutils/utils/io/impl/YangFileScanner.java
+++ b/src/main/java/org/onosproject/yangutils/utils/io/impl/YangFileScanner.java
@@ -45,7 +45,7 @@
      * @throws IOException when files get deleted while performing the
      *             operations
      */
-    public static List<String> getJavaFiles(String root) throws NullPointerException, IOException {
+    public static List<String> getJavaFiles(String root) throws IOException {
 
         return getFiles(root, JAVA_FILE_EXTENTION);
     }
@@ -59,7 +59,7 @@
      * @throws IOException when files get deleted while performing the
      *             operations
      */
-    public static List<String> getYangFiles(String root) throws NullPointerException, IOException {
+    public static List<String> getYangFiles(String root) throws IOException {
 
         return getFiles(root, YANG_FILE_EXTENTION);
     }
@@ -73,7 +73,7 @@
      * @throws NullPointerException when no file is there
      * @throws IOException when files get deleted while performing the operations
      */
-    public static List<String> getFiles(String root, String extension) throws NullPointerException, IOException {
+    public static List<String> getFiles(String root, String extension) throws IOException {
 
         List<String> store = new LinkedList<>();
         Stack<String> stack = new Stack<>();
@@ -100,8 +100,8 @@
                 }
             }
             return store;
-        } catch (NullPointerException e) {
-            throw new IOException("NullPointerException occured");
+        } catch (IOException e) {
+            throw new IOException("No File found of " + extension + " extension in " + root + " directory.");
         }
     }
 }