Improve warn log to print more information

[deshankoswatte]

Purpose

Improve warn log to list the directory where the files were null.

Goals

With this improvement we can easily identify an issue without the need of a log patch.

Security checks

• Followed secure coding standards in http://wso2.com/technical-reports/wso2-secure-engineering-guidelines? yes
• Ran FindSecurityBugs plugin and verified report? yes
• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes

Related PRs

#270

Test environment

JDK 1.8, Ubuntu 20.04.3 LTS