Check through code base for use of "magic numbers"

[jrfnl]

Most notably in the cURL transport class, hard-coded numbers are used for various cURL error codes.

Task:

• Go through the code base to find these numbers
• Check if there is a PHP constant which can be used instead and if that constant is available in all supported PHP versions and if so, switch over
• For "magic numbers" which don't have a PHP native constant, create a descriptive class constant instead and use that.

[jrfnl]

Relevant links:

• https://curl.se/libcurl/c/libcurl-errors.html
• https://www.php.net/manual/en/curl.constants.php

[jrfnl]

PR ready. Will pull after #519 has been merged to prevent merge-conflict hell.

[jrfnl]

All related PRs have been merged. Closing the issue.