fix lxc_error_set_and_log to match the docs

fix lxc_error_set_and_log to match the docs

The documentation for this function says if the task was killed by a
signal, the return code will be 128+n, where n is the signal number. Let's
make that actually true.

(We'll use this behavior in later patches.)

Signed-off-by: Tycho Andersen <tycho@tycho.ws>