docs: add "Error Handling" in "Upgrading from 3.x to 4.x"

[kenjis]

Description
From https://forum.codeigniter.com/showthread.php?tid=88993

• add "Error Handling" in "Upgrading from 3.x to 4.x"

See

• https://github.com/bcit-ci/CodeIgniter/blob/develop/system/core/Common.php#L600
• https://github.com/codeigniter4/CodeIgniter4/blob/develop/system/Debug/Exceptions.php#L202

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide

[paulbalandan]

At first glance, I don't quite understand it well. Can we make it in tabular form?

[kenjis]

The production setting is error_reporting(E_ALL & ~E_NOTICE & ~E_DEPRECATED & ~E_STRICT & ~E_USER_NOTICE & ~E_USER_DEPRECATED). So if E_WARNING error happens, in CI3 it is logged, but does not stop the framework. But in CI4, it is logged and stops the framework.

After all, this seems okay.