refs LibraryOfCongress#123 - fixed error message formatting

src/main/java/gov/loc/repository/bagit/exceptions/MissingPayloadDirectoryException.java

@@ -1,12 +1,16 @@
 package gov.loc.repository.bagit.exceptions;
 
+import java.nio.file.Path;
+
+import org.slf4j.helpers.MessageFormatter;
+
 /**
  * The payload directory is a required file. This class represents the error if it is not found.
  */
 public class MissingPayloadDirectoryException extends Exception {
   private static final long serialVersionUID = 1L;
 
-  public MissingPayloadDirectoryException(final String message){
-    super(message);
+  public MissingPayloadDirectoryException(final String message, final Path path){
+    super(MessageFormatter.format(message, path).getMessage());
   }
 }

src/main/java/gov/loc/repository/bagit/verify/MandatoryVerifier.java

@@ -84,7 +84,7 @@ public static void checkPayloadDirectoryExists(final Bag bag) throws MissingPayl
     final Path dataDir = PathUtils.getDataDir(bag);
 
     if(!Files.exists(dataDir)){
-      throw new MissingPayloadDirectoryException(messages.getString("file_should_exist_error"));
+      throw new MissingPayloadDirectoryException(messages.getString("file_should_exist_error"), dataDir);
     }
   }