nfs/Connection: nfs_get_fd() can return -1

While reconnecting to the NFS server, the socket is -1, because there
is no socket.  Fixes a potential crash (assertion failure).
nfs/Connection: use only BroadcastError()

Since BroadcastError() calls BroadcastMountError(), there's no need to
check mount_finished here.
nfs/Connection: check for disappearing libnfs socket