ui: login: do not show connection failure as login failure

ui: login: do not show connection failure as login failure

In case pvedaemon, pveproxy is not running or we get somehow else a
595 response (connection failure), we want to tell this to the user,
since "Login failed" doesn't help.

Handle all connection failures, leave the rest the same. We could go
into more detail (CONNECT_FAILURE, CLIENT_INVALID, LOAD_FAILURE and
SERVER_INVALID), but i didn't see any reason to make all these
distinctions now.

Signed-off-by: Oguz Bektas <o.bektas@proxmox.com>