Fix cryptic callback response message when IP hasn't been whitelisted

[danilhendrasr]

Description

When receiving callback, if the user's server IP address hasn't been whitelisted, Xendit will get back cryptic error message that doesn't show what's wrong:

One has to dig through the code, adding logs manually, to find the root issue:

The process of finding the root cause can be unnecessarily time consuming. With this PR if the user's server IP hasn't been whitelisted upon receiving callback, Xendit will get back a clear response message pointing out what's wrong:

Thus users have clear indication of what's wrong just by inspecting the callback log from Xendit dashboard.

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] Cosmetic change (text changing or color changing)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code (if necessary), particularly in hard-to-understand areas
• [x] My changes generate no new vulnerabilities
• [ ] I have added tests that prove my fix is effective or that my feature works

[yanoandri]

Hi @danilhendrasr thank you for raising this, will merge this PR to solve the issues, thank you for improving our Magento plugins