wp_die() updates for the Upgrade / Install component

[CallumBW95]

Update wp_die() to include status codes where appropriate within files related to the Upgrade / Install component

Trac ticket: #64058

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[mindctrl]

I wonder if 404 is the correct code here. The client doesn't request the wp-config-sample.php file directly. The backend tries to load it but can't. I could see this being a 500, but curious to hear other opinions.

[CallumBW95]

I have done some reading around this, and it's definitely not a client issue, so a 4xx error would not be correct. I believe this one can be turned back into a 500 error as this is due to a missing file on the server. I will update this now.

[mindctrl]

Looks good overall. Left a thought about the 404 code in setup-config.php.