Print a message when failing to open a non-socket device

Ensure a friendly message is printed regardless of the type of device instead of bailing out silently when the opened device isn't a socket, which confuses the user. This restores the pre-2.10 behaviour.

Fixes: 095583d2 ("Rework device file locking") Closes: #14 (closed) Signed-off-by: Yao Zi ziyao@disroot.org