upgrade.php: More helpful output when we can't read .upgrade_requirements.json

[jerm]

Handle json decode errors and display the raw output in such cases, to
better understand what's happening and help debugging

[what-the-diff]

PR Summary

• Improved Certificate Handling on Windows
  The code has been enhanced to cater loading of intermediate certificates on Windows platform if the situation demands, making the application more flexible.

• Modified Method for Upgrade Requirement Retrieval
  The strategy for pulling the upgrade requirements from GitHub has been amended which is expected to increase the efficiency and accuracy of the operation.

• Enhanced Error Handling for JSON Decoding
  Introduces robust error handling procedures for situations of JSON decoding failures, thus increasing the reliability of the software.

• Updated and Informative Error Message
  The error messages have now been upgraded to provide more detailed information about the issues and also suggest potential fixes. This enhances user experience by facilitating easier troubleshooting.