Replace trigger_error(E_USER_ERROR) with exceptions (PHP 8.4 compatibility)

[lacatoire]

Summary

This PR replaces deprecated usages of trigger_error(..., E_USER_ERROR)
with proper exception handling.
As of PHP 8.4, E_USER_ERROR is deprecated and will be removed in a future release.

Motivation

• Ensures forward compatibility with PHP 8.4 and beyond.
• Improves error semantics and debuggability.
• Replaces legacy fatal error triggers with modern exception-based handling.

Scope

• Replaced trigger_error(..., E_USER_ERROR) with:
  • throw new RuntimeException(...) for runtime failures (I/O, missing dirs, etc.)
  • throw new Error(...) for programming or internal state issues
• Left E_USER_WARNING / E_USER_NOTICE untouched.

Example

Before:

mkdir($this->userNotesBaseDir, 0777, true)
    or trigger_error("Can't create dir: $this->userNotesBaseDir", E_USER_ERROR);
After:
if (!mkdir($this->userNotesBaseDir, 0777, true)) {
    throw new \RuntimeException(sprintf(
        "Can't create the usernotes directory: %s",
        $this->userNotesBaseDir
    ));
}