page-footer: clarify exit code 25

page-footer: clarify exit code 25

- Clarify that curl tool exit code 25 means an upload failed to start.

Exit code 25 is equivalent to CURLE_UPLOAD_FAILED (25). Prior to this
change the documentation only mentioned the case of FTP STOR failing.

Reported-by: Emanuele Torre
Ref: https://github.com/curl/curl/blob/curl-8_4_0/docs/libcurl/libcurl-errors.3#L113-L115

Fixes https://github.com/curl/curl/issues/12189
Closes https://github.com/curl/curl/pull/12190