Message file verifier: allow to simply enter the language code instead of full path

- To check a single file just enter the language file, e.g. "es"
- Pass File object to MessageFileVerifier instead of String path that will be constructed to a File again...

src/test/java/tools/messages/translation/ImportMessagesTask.java

@@ -89,7 +89,7 @@ public class ImportMessagesTask implements ToolTask {
             throw new IllegalStateException(e);
         }
 
-        MessageFileVerifier verifier = new MessageFileVerifier(fileName);
+        MessageFileVerifier verifier = new MessageFileVerifier(file);
         VerifyMessagesTask.verifyFileAndAddKeys(verifier, YamlConfiguration.loadConfiguration(
             new File(MESSAGES_FOLDER + "messages_en.yml")));
     }