curl: exit on config file parser errors

Like when trying to import an environment variable that does not exist.

Also fix a bug for reading env variables when there is a default valu

curl: exit on config file parser errors

Like when trying to import an environment variable that does not exist.

Also fix a bug for reading env variables when there is a default value
set.

Bug: https://curl.se/mail/archive-2024-02/0008.html
Reported-by: Brett Buddin

Add test 462 to verify.

Closes #12862

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