fix: filter out mirrors with failed curl connections in rank_mirrors()

[MykolaVaskevych]

Summary

• Check for any non-zero curl exit code, not just timeout (28)
• Only accept HTTP 200-299 instead of rejecting >= 400 — catches status 000 from broken mirrors that accept TCP but return no valid HTTP response

Fixes #184

Test plan

• Set mirrorlist URL to UK (?country=GB — mirror.marcusn.net is currently broken)
• Refresh mirrorlist via widget
• Verify broken mirrors are excluded from /etc/pacman.d/mirrorlist

Tested against real mirrors:

Unreachable - https://mirror.marcusn.net/...  (exit=0, response='0.046616 000')
Unreachable - http://mirror.marcusn.net/...   (exit=0, response='0.046763 000')
OK 0.089373s - https://london.mirror.pkgbuild.com/...  (exit=0, response='0.089373 200')
OK 0.083120s - https://archlinux.uk.mirror.allworldit.com/...  (exit=0, response='0.083120 200')
OK 0.095233s - https://mirrors.ukfast.co.uk/...  (exit=0, response='0.095233 200')