Since BroadcastError() calls BroadcastMountError(), there's no need to
check mount_finished here.

While reconnecting to the NFS server, the socket is -1, because there
is no socket.  Fixes a potential crash (assertion failure).

Necessary for "final" on gcc 4.6.

std::map::emplace() is only available from gcc 4.8 on.

More robust and cancellable.